I was a high school 4:44 miler.
At age 35 I ran 5K in 16:04, age 48 16:41, age 52 16:38.
I have done two significant things from my late 40s to now (54): (1) October-March patient aerobic base building on hilly, soft trails and flat grass w/long weekend trail runs progressing month-by-month at 135->145->155->160bpm HR and (2) I learned to use the power in my spine to deliver power to a deep, light footstroke with a nearly flat shoe. A 4cm heel drop is 10-15 secs/mile faster than a 8cm or 10cm heel drop, because every time the heel bottoms out, forward momentum is checked.
I'll leave you my poem, my mantra.
5,000 Meters
5,000 meters
No limits
No limits
No limits
Faster and faster
Make time stand stil
5,000 meters