>"Marathons aren't high intensity workouts."

It sounds like you've never run one. World class athletes can run an entire marathon at a sub 5m mile pace. While they could run a single mile race in 4:05 or so, it's not even remotely a low intensity effort to run a 5 minute mile, even for the fittest people on the entire planet.

Unless you've put real effort into training, 60+ year-old competitive marathoners have done a 20 minute tempo run faster than you ever have... and repeated it 7 consecutive times without rest.

Marathons are high intensity and extended efforts.