I've read numerous times that the 7-10 year improvement 'arc' is real and I believe it. For many of us, beyond simple burnout and temptation to wander into other hobbies/endeavors, there are the issues of recovery and healing.
In my late 40s my ability to recover from hard workouts declined. From my early fifties it seemed that every little tweak would slow me down or halt training for more than a week at a time, coupled with a serious reduction in average pacing of runs. I moved more into other areas including cycling, rowing and skierg, not because I really wanted to but because I needed to be able to exercise without constant frustration. The pounding of running really becomes noticeable and pronounced with age.
This February I decided to become aging ultra guy, which would allow me to train very, very slowly and try something new. I slowly worked up to 60mpw at very, very slow paces and was doing great, until a mid-fifties thing got me again. Long run, 14 easy miles on the treadmill, easy pace, felt great, and then next day felt 'something' in my left hip. Of course, that 'something' has stuck with me and I'm temporarily back to skierg and rowing with some gentle walking until my hip decides to cooperate again, assuming it does.
THAT's the problem many of us face as we age. It's not a matter of want, or dedication, it's the constant niggling tweaks that may or may not become injuries, that keep us off a plan, for weeks and months at a time, no matter how smart we train, no matter how careful we are. That's the thing for me. Every time I come back to running now, after decades of high mileage and no injuries that ever sidelined me, I can't build effectively without setbacks.