I ran 16 marathons in 2016 as well as a 24-hour event where I completed 105 miles, then ran the Chicago marathon less than two weeks later.

I never felt healthier in my life than that year.

This is not medical advice, but the body is amazingly adaptable and can withstand some pretty high levels of stress if you build up to them slowly and are willing to back off when it tells you to.

I do two hours of moderate cycling every day and I feel like fuck.

Of course you gotta also consider how many under train for the marathon and essentially over commit themselves to the event when it does come along. But it definitely isn't healthy if they require more than a day or two of physical recovery after a physical event.