re: 10-week 5k training plan (under 20min)

Definitely follow the advice in the previous post and focus on speedwork. From my running experience one of the main reasons why people significantly slow down time wise, or get injured more as they age is they don't focus enough on speed (track) work.

I honestly don't know any 40+ year olds who haven't been running fast consistently since their 20s running a 20 minute 5k, but I'm rooting for you, good luck!