The main question had been about running with a ruck, and I'll get more into the ruck-specific aspects later, but I'll focus on the running part first, because as the second part of the question highlighted, the biggest factor to me is general running or hiking endurance. If your legs are strong and your aerobic engine is strong, it'll take you a long way.
1. Don't get injured / listen to your body / strong hips/quads are a girl's best friend
When I first got "more serious" about running while in college (far too slow to make any team, so it was just running on my own, 6 mi, 2x/week, on average), I thought "no pain, no gain". I wanted to be able to show improvement on each run, so I tried to PR on every training run. I was really enthusiastic, and would fall asleep listening to running podcasts like old-school Phedippidations. However, I ended up developing knee pain that limited how much I could run. I had the heart, but my body wasn't cooperating.
After college, a PT friend helped determine that I had really week hips, and once I started strengthening my quads with body weight lower leg extensions (placing a pillow or foam roller under my quads, and just raising my lower legs), and body weight hip abductions and adductions while lying on my side, and doing single-leg squats, my knee issues went away. I later read that knee issues can be common, especially among women, and that the hip exercises could be magic. I still do some of those exercises, and as little as one set of each exercise once or twice/week made a lot of difference.
Everyone may have different imbalances, but that was mine. Once I addressed it, I've been able to stay pretty much injury-free for the past 8 years. There are still small nagging issues every now and then, with my knees, or my plantar fascia, or the occasional mild ankle roll, but nothing that takes me out of training... I tend to rest 1-3 days between workouts anyways, so by the time I'm going to do another workout, it's usually better, or at least good enough to continue with, anyway.
That leads to the topic of rest. I do less mileage, and take more "off days" than most, probably. I feel like my body just needs more time to recover. If my legs feel "used" or "swollen", I could push it, but usually don't. Sometimes, it's merely a matter of getting blood flowing, and they're really ok to run on. But other times, if I'm just not feeling it, I'll make it a rest day. A couple times, I've tried pushing through and working out anyways, but it typically ends up in having one of those nagging issues pop up.
Mileage-wise, I do far less than most who do races in the times/distances that I do them, but it works, and it's what I feel like I can safely handle. I'll get more into my typical workout week in the next section. But the key is, I try to listen to my body, and am not afraid to take rest days.
I've had long periods of being inured in high school and in college, not only with the knee pain, but also with ankle rolls that would take me out for about 3 months each time. Being injured is no fun. The physical pain is nothing, compared to the mental struggle of not having exercise as an outlet and races to look forward to. You find ways to cope. When I'd be out for 3 months after an ankle roll, I'd replace my usual run workouts with body weight strength exercises... lots of core work.
In racing, being 10% under-trained is far better than being 1% over-trained.... that's the old adage. In training, it's something similar... if you run yourself to injury, and you end up being out for X weeks, your fitness will drop. If you temper your buildup and listen to your body, give it time to adapt to small incremental increases in training stress, you may not meet your full potential in the shorter run, but perhaps in the longer run, you'll have far more months and years of cumulative training. Endurance built up over many years is for real. There are others I've heard of, who can ramp down their mileage a bunch, but their performance doesn't drop much at all, because of cumulative endurance. Also, as people get older, they should be getting slower, but I'm still getting faster, for the most part... very incrementally, but I do think I am getting faster still, because of the continued accumulation of endurance. There are so many adaptations that take place, many of them undetectable (like capillaries growing, to better deliver blood to muscles)... it all adds up over time.
So, don't get injured, so that you can get more training in the long run, listen to your body.
2. My typical training week
Most of my runs are 5-6 miles, medium effort. I'll have a longer run once every 1-2 weekends, ranging from 8-20 miles, depending on whether I'm training for a marathon, or in maintenance mode. Every now and then, when I am training for a specific road race (anything from a 5K to a marathon), I'll do intervals. My bread-and-butter inteval session is a 1 mi warmup, and then 3 x [0.75mi or 0.5mi fast, then jog 0.25mi]... where "fast" is about 5K race pace.
When I'm in maintenance mode, I may do only 10-15 miles/week (2 x 5 mi run, or 1 x 5 mi and 1 x 8 mi). When I'm training for a marathon, I max out at 25-30 miles/week. This is far less than most, but it's what I feel like I can handle, and I've gotten away with it. Some people are able to handle more mileage better starting to risk injury, than others.
The #1 thing that people can do to improve their marathon times is supposedly more mileage. I often tell myself "next training cycle, I'm going to try to up my mileage by adding in more slow runs to safely add volume". But I haven't actually mustered the energy to really try it yet. The more risk, the more reward. So if I really want to get past my plateau (I've been running similar marathon and 5K times for the past like 6 years), I should try something different... but then you have to make sure you're doing it safely. Work can make me tired, too... so I haven't tried it yet. I don't know how people with families to take care of do it all. What they're able to achieve while juggling it all is incredible. I feel like I'm barely capable of taking care of myself sometimes, between cleaning the house, dishes, laundry, etc.
During marathon training, the long run is the key, but I still periodize that, taking a big cut in mileage once every 3-4 weeks. Becoming stronger doesn't come during the time you're exercising. That's when you're actually damaging your body and therefore getting weaker. It's during the resting time that your body makes its adaptations and gets stronger.
During the long run itself, and for building endurance, going longer is more important than going fast. I'll intentionally go super slow at the beginning of my long runs, because it guarantees that I'll make it through the full distance that I'm planning. Negative splits (gradually increasing your pace during the entire run, by starting slow and tempering your effort so that you don't burn out in the beginning or mid-way) are how I approach all of my medium- and long- runs. Finish feeling good. For short and fast runs like intervals, I keep the pedal to the metal the whole time, still not killing myself the first rep, but the reps naturally get harder as you go on, even for the same pace.
Once per week, on average, I also do a 1-hr body weight strength session. Some push-ups (my upper body is far weaker than the rest of my body), lots of planks, lots of sit-ups, lots of hip-related exercises, squats, started adding lunges (it's supposed to simulate hiking up a steep mountain, and I agree... but my knees need to feel 100% strong to pull them off), and some 10-lb dumbell exercises. It feels really good to do the body weight strength session. It fixes imbalances that might've emerged from the repetitive and linear motions of running, and from sitting all day at work. It gets blood flowing and helps with recovery. Afterwards, it always feel like I've given my body a "reset" that will make it good to go and injury-free for another week. I can feel it when I go more than 2 weeks without it.
I'll sometimes substitute a run with, or supplement running with a day of stationary biking per week. It gets blood flowing through my body, helping with recovery. It gets in a little training effort, with minimal impact. I do go to the point where my hips get tired/sore, which may not be ideal for the run I do the next day or the day after, but I still through it in once every 1-2 weeks.
THOUGHTS ON RUCKING
1. The Hip Belt
There's debate in the community about the use of hip belts. Those from a military background are pro-shoulder carrying without the hip belt. Those from a hiking/backpacking background are pro hip belt. I totally get the hip belt. Your legs are going to carry the weight anyways, regardless of whether it sits on your waist or your shoulders. By putting it on your waist, though, you get to bypass putting any stress on your shoulders/upper body. Maybe it's just because I don't have a strong upper body, but having the weight go directly to my lower body, instead of sitting on both the upper body and transferring to the lower body, has felt more comfortable.
I was curious about why some prefer the shoulder carry. I tried to google it, and this is the only answer I've found so far (see minute 4:45):
Disclaimer: I'm not a big backpacker, and have only done 1 week of backpacking, but the time I did to it, and the times I've worn rucks (GORUCK Tough, HH12HR, BEL), having a waist belt cinched tight around my body has made me feel much better. It could be the design of my particular ruck / my particular body, but it's worked for me.
I haven't been able to justify paying $45 for a GORUCK hip belt with my GORUCK GR1, so what I use instead is a $2 strap with clips that I bought at Walmart (it's not meant to be a waist/hip belt... it's just an all-purpose strap with a buckle, that I found in the camping section). I attached it to the MOLLE on my ruck. It's around my waist rather than my hips, technically, but it still allows the weight to go more directly to my lower body. It's not equivalent, I'm sure. It's a thin strap, and is not padded at all. I don't chafe, though, and it feels much better than having nothing at all. Worth a try, for $2, at least to see what having one would be like. That $2 strap has survived 3 endurance events so far, plus the mini training, which I'll get into next.
When I say "mini training" with the ruck, it's mini, because it's mostly just me walking to and from the fitness room (0.5-0.9 mi 1-way, depending on whether I want to add extra distance to get a Pokestop). I'll also wear it when I go hiking with my dad, which is about once every 2 months. I tend to get pretty tired of having that weight on my back (in spite of the hip belt) after about 6 miles. Maybe that says something about the limitations of my $2 hip belt... would a $45 padded hip belt be better? Or is the shoulder carry the way to go? Does a hip belt and shoulder carry need to be mutually exclusive? No. I think hikers also recommend having the heaviest objects high in the pack, and closest to the body, along with having the hip belt. Or, it could be that I'm just not strong enough yet. Or a mixture... point is, I get tired after about 6 miles of hiking with a ruck.
2. Running with a Ruck
The big secret is that I don't train running with a ruck. I only walk / fast walk with the ruck, a couple times a week. Running entails leaving the ground, bouncing up and down. That's a lot of stress on your knees. Using 20LB like I do, it's an extra 20LB slamming down on your knees with every step.
I save running with a ruck for the actual event. I have no issues with suddenly being capable of doing that on the day of the event, because of the running endurance and being at least moderately used to wearing the ruck during the short walks a couple of times per week.
When I do move, whether it's while walking or running, I try to minimize my vertical oscillation (minimize bounce). I feel like that's another area where the hip belt helps. It keeps the ruck tight against your body, so that it moves with your body, rather than bouncing all around. Even without a hip belt, making your shoulder straps as tight as you can against your body also helps to reduce bounce. Minimizing bounce means doing something more reminiscent of a shuffle. Not only do you avoid the bounce... you also avoid the extra work of having to raise and re-raise the weight with each step / mini-jump that running entails.
Disclaimer: I don't ruck that often, and not for that long of a distance, but what I mentioned above has been good enough strategies for helping me survive what I've set out to do so far. If I were to take on longer events, multi-day events, heavier events then maybe I'd need another strategy or different gear... who knows. But for the 12hr events, it's worked well enough for me. Also, I've only ever tried one brand... GORUCK. Perhaps there are other brands that would completely change my view if I tried them... but I can only speak for what I have tried. I had invested in a GORUCK (during a big sale), because of its durability and reliability, and haven't been disappointed. Again, other brands may be just as good, and have their own features and merits that make it even better than GORUCK in some ways. I might try a different brand one day... my GORUCK is bright red, which isn't the best, when you're trying to be stealthy ;) It could be a good excuse to get another rucksack from a different brand and branch out.
Most girls have a large collection of purses... I have a large collection of backpacks.
Most girls have a large collections of heels... I have a large collection of running shoes. ;)