I'm 51, been running since 1979, more than 100,000 lifetime miles. On track for just more than 3,000 this year.
What feels most different for me is much less of a feeling of being capable--that I can go out any day and execute whatever it is I've told myself I'd like to do that day. How I'll feel, and where within the range of what I can run for pace and distance is much more unpredictable now. And the consequences of doing something like sticking with an interval workout or long run on a day when I don't feel great are much greater--it's much easier now to realize a few days later "I've been dragging since Thursday, and I know exactly why."
That can make knowing how to proceed more difficult: Am I just a little off and should suck it up and finish the workout as planned, or am I getting legitimate signals to call it a day?
The greater range of day-to-day feeling, and the unpredictability, make racing less satisfying. I can no longer do X, Y, Z and be reasonably sure that will mean feeling ready to go on race day.
Subjectively, my motor doesn't rev as high. By that I mean what feels most comfortable is more often toward the easy end of the effort scale. It used to be more toward the medium to steady state end for daily runs. I finish very few runs with that run-to-the-barn feeling of everything clicking; they used to happen a couple times a week.
Mentally, running regularly is much easier than it used to be. I have that much more experience of getting out the door regardless of what's going on, and, perhaps because of age, I have the wisdom to know any excuse I come up with that's not injury or illness is almost certainly BS, and that at the end of the day I'll be happier for having done even a token run.
One change over the last decade is that morning runs feel easier than they used to. I used to do a short run before work, then my main run, including workouts, after work. Doing a 10-miler before work was a big deal, and a workout was unheard of. Now when I try to do something longer or harder at the end of the work day it doesn't go well, even if I've done nothing in the morning. I really like this change.
Mechanically, I don't feel much different, although I know I look worse. I most often feel noticeably worse when I haven't run in the morning and have sat in front of the computer for too much of the day and then go run, even if I do a good stretch before the run.
I always loved running, but now I appreciate it more--all the things that initially attracted me still exist, and it adds a layer of pleasure to my middle-aged life that isn't there for most of my contemporaries. One aspect of the latter is that, thanks to getting much slower, I have a broader range of potential training partners. This makes for more varied conversations and social experiences than I would otherwise have as a 51-year-old man.