My best friend died at 24 from a fluke heart attack. He was in shape and had no prior health issues beyond mild asthma as a kid and needing to wear glasses. He just collapsed one evening, stopped breathing within a few seconds and never regained consciousness.

He didn't do drugs. He wasn't overweight. Hell, he barely drank alcohol. He had never had any kind of heart issue noticed in routine physicals and definitely didn't have any symptoms that set off alarms for him. He certainly didn't do anything nearly as physically taxing as running a fucking marathon.

He died in 2005. I can probably find the fucking prayer card if you'd like.

If anecdotal evidence isn't enough, here is an article written in 2019 discussing a rise in heart attacks among people in their 20s and 30s. This shit has always fucking happened. Even a low incident like like 3-4 per 100k will result in that happening to THOUSANDS OF PEOPLE every year, most of whom are not engaging in incredibly strenuous physical activity like a marathon.