When I was younger, I was a lifeguard. I could swim pretty hard for a couple hours if pushed. I used to swim 2 miles at lunch every day for fun. What I couldn't understand was on the high school swim teams, I knew plenty of people that swam sprints, but their coaches would make them swim lap after lap every morning. I know it was to build stamina, etc.... but if I'm gonna sprint every weekend, I'm not going to swim hard endless laps for an hour every day. Funny thing was, I could keep up with all my sprinter friends. Distance guys could beat me easily.
Similar thing happened with our kid that ran cross country. They have to run miles and miles every day but only ran 5K in the meets on weekends. Our kid had a bunch of personal and job related things come up, so had to cut practices back to only 1-2 a week. Then her times at the meets started going up. She set personal records about 6 weeks in a row. Coach asked what was up? I said I thought he was running them too much during the week, and explained the reasoning. She ended up taking 1st place JV in the city meet.
I'm sure it's different for elite athletes, which I nor my kids are not. They have that stuff down to a science at some places. But for the average person, I think it's easy to overdo it, thinking you're doing yourself some good.