I'm 61 now. I ran consistently from age 28 to 35. Took some time off to raise kids, and then started running again in my mid-40s, and started running 50 miler ultras at 50. Backed off to avoid injuries but still running fairly consistently with many months off here and there due to injuries.  While I was never fast I did notice that I did progressively go slower over the years.  It's inevitable.

Now I just run 15-25 miles per week at a very slow pace, using the Maffetone method just to keep my aerobic base going.  I recommend this approach.  You just go very slow but go long, >1 to 2 hours to keep building you base w/o injury.  Since adopting Maffetone I have been injury free for several years and I feel great and I love my runs.  I also love that strenous hikes count as maffetone training.

There is no need to run fast.  Just stay in your aerobic zone. Google it up.