Disc golf can’t be that tiring, right?

Over the J4th weekend, I had family come visit us. I took my brother in laws and teenage nephews out to play for the first time at a relatively easy course. Now here’s the thing, my brother in laws were top basketball players in their younger years and are still quite athletic for their age. My teenage nephews played lots of baseball during the spring. So all of them are quite athletic and fit, more so than me. But we were only 9 holes in out of 18. The nephews dnf. One of my brother in laws was cramping. The other brother in law kept up, but just barely. We did finish, but they were wayyy more exhausted than what’s normally expected. Is this normal for first timers? (then again the temps were in the high 90s)