I took about 15 years off racing between age 44 and 59, and have run a few 5k races since. I continued to run during that break, although at much lower volume tan previously. Over a year and a half of race attempts I gradually improved from 20:03 to 19:28, and hope (!) to improve a little more next month on a very flat course at 60 years and 7 months.
The main difference for me (just one data point) between training and racing at 59-60 versus 44 is the difficulty managing reasonable volume and workouts without risking injury. While I used to average about 65 mpw, sometimes getting above 100, i now usually run about 25 and might see 35 occasionally. With a couple of workouts a week, thats about my personal limit.
I’ve kept the workouts very simple, typically rotating between 400s at close to mile pace, 1000s close to 5k pace and longer stuff (maybe 2000s at close to 10k pace or longer strong “tempo”).
I have not personally (yet, anyway) managed to reach equivalent times from my younger racing days; the 19:28 was about 82% age-graded (2015 WMA tables) versus 84-87% across the range from 1500-HM at 42-44. It strikes me that regaining your youth, in age-graded terms, is very difficult, at least for me.
Best of luck in the attempt. I hope it goes better than Henry Rono’s public attempts on here a dozen or so years ago.