Yeah I can see a treadmill being extremely boring. I usually ride and run outside, but I had to ride on a trainer for a couple of weeks because of an injury and it was just awful. I couldn’t wait to be done. But when I’m outside sometimes I’ll extend my ride for another 10km just because the weather is nice and I feel good, I’m (almost) never in a rush to finish.
I don’t think I could run on a treadmill either, at least outside I have short goals to reach (ok now to the next lamp post…)