Years ago I went to a gym and walked on a treadmill a lot. I'd rather be walking in the woods but when everything is covered in snow it's not such a good idea and when the temperatures run north of 100 it's a struggle to physically carry enough water for a decent hike. The key thing is finding a way to keep cool in the summer - I have a stationary bicycle as well but haven't used it in ages because it's tucked out of the way, and being out of the way means there's not much air circulation. On a real bike the harder you push it the more wind resistance you face, which helps keep you cool. On a stationary bike the same power gets you a higher notional speed because the air resistance isn't there, but it also means you get very hot very fast, which isn't much fun.
