It's been raining all week (and probably will continue to rain for the next few months) up here in Oregon. I've done some runs in the rain, but I tried my first long-ish run on a treadmill ever. I don't think I've ever done more than a mile or two on a treadmill, but I did an hour. I didn't even note the exact "mileage" at the end, because it seems like treadmill mileage isn't real. I was just going for an hour and keeping my heart rate above 150.
I've always thought of treadmills as easier, but after this long run, I realize they are just different. Maybe a high-end treadmill would be another story, but I'd say I got a better workout in this 60 minute run than I would running outside, because it forced me to keep up the pace. It's also much harder to keep going when you can just stop at any time. When I run outside, I always run away from my house, so that about the time I'm wishing the run was over, I have no choice but to keep running to get home.
Only thing I really don't like about the treadmill is the little bit of slipping and shifting when my foot lands. As a bigger guy, I've always felt that, a little tug on the motor and the belt sliding. It's just for a fraction of a second, but I felt myself using extra effort to stabilize as I ran. Good news is that my knees and ankles felt a bit better than usual after.