A former co-worker was on the track team in high school, and his event was the 400. One year he was following the Boston Marathon on line because a neighbor was running. It occurred to him that the average pace for the leaders was faster than his PR for the 400. In his prime, he couldn't have kept up with those guys for a quarter mile.
On the other hand, one time when orienteering at Round Mountain, I remember zooming past Peter Snell (I was on Blue, he was on Brown? Green?). He once ran 800 in 1:44 on grass(!), and also had the mile world record at 3:54. It occurs to me now that I'm almost as old now as he was the day I passed him.
I've run 400m in 70s, which would be somewhere close to those marathoners, but after doing that for only 400m I felt like my lungs were on fire for the next 45 minutes. It's amazing how fast some humans can run for 26 miles!
You can do it! Live healthily, train hard and rest well.