Hoping someone else might benefit from my recent experience.
I'm a 30-something dude, fit, been running for many years but the past two years more seriously, always plagued by overuse injuries (stress fractures, runner's knee, etc.) and thus have taken a lot of time off. Just this year I've finally gotten to a place where I can run 15 MPW comfortably at around 8:30 pace, with little fear of injury, with a combo of stretching, foam rolling, decent diet, weekly strength training, etc. I have been doing this pretty consistently for a few months now and I feel great.
Never one to be satisfied with a good thing, I want to train for a half marathon, but I haven't run over 4-5 miles straight in a few years. (I did a 10K with zero training about 10 years ago...) Every time I've tried to ramp up my weekly mileage, or extend my "long" runs, I get injured. I can easily do 7:30 for 3-4 miles, but I often end up going ~8:30 pace, since I shoot for 9:00 to be safe. I do feel pretty beat up after 4-5 miles. Not so much in oxygen debt as joint and muscle fatigue, aches and pains, etc. I don't really feel like it, but my HR gets up to 170-180 by the end, which is confusing since my resting HR is 50-52. Previously I thought I just had a naturally high MAX HR, like over 200.
I started reading about low HR training (you can read about it elsewhere--basically running most of your miles REALLY easy to improve aerobic base). While I don't necessarily buy all the hype surrounding it, I decided to try today running 4 miles tracking not my pace, but my HR, just to see how fast it went up. After warming up, I noticed it was about 145, so I decided to try not to let it go over 150. I ran slower, tried to relax, and every time it jumped a little I slowed down a little. I ended up running 4 miles at an average HR of 140, which was about 10:00 pace--and it felt SO EASY, like I could go forever at that pace. It was like nothing.
So I'm officially a convert to tracking HR instead of pace, at least for now. Supposedly if I do this long enough my pace will increase while my HR stays the same? I don't really care how fast I run; I just want to be able to finish a half and then (someday) a full marathon, and if it's at 10:00 pace, I'll be happy. It's been a dream of mine I never thought I'd do, since I'm older now, but this has given me a new hope.
TL;DR: If you're having trouble with overuse injuries or feeling like you hit a plateau, try tracking HR instead of pace and keep it