I chimed in a bit on the first page; not exactly replying to popular demand here, but here's a summary of regression and response to aging.
Been running for over 30 years, peaked mid 20s to early 30s (mid-low 15s for 5K), declined rapidly during late 30s (to high 16s), fought back for a few years (mid 16s) during early 40s, fell apart until mid 40s (18s, but barely trained), fought back again late 40s and early 50s (low 17s).
In my early 50s I'm running better than at any time since 42.
Things that have worked:
good base throughout the year, including substantial cross training. I train about 400 hr/year, with decent mileage through running season (50s to 70s/week).
going very easy on recovery days
modifying V02 type workouts, doing 50 to 70% of workout and not hammering, just getting some specific training
consistenly adding speed work, but very short sessions (1 mile or less/week)
consistent tempo runs
maintaining long runs/workouts throughout the year (1.5 to 3 hr)
form drills