I find treadmill running to be harder than road running just because it is so incredibly boring. But as far as difficulty, I think the hilly roads around my neighborhood are much harder to run on than a treadmill, even if I make the treadmill do hill intervals. A treadmill tends to propel you a little bit, plus if you are trying to maintain a certain pace, obviously this is not automated on the road the way it is on a treadmill, so you have to be more tuned in to what you are doing.

I, like you, have always been one of those people who thought I was not built for running, but I've always done a little of it on and off. About a month ago I started triathlon training and found that I really enjoy running now much more than I ever have. Tomorrow I have five miles planned and I'm actually looking foward to it.