Random11

Agreed. Thinking about this, one thing that makes running "harder" than cycling is that it is harder on your joints. I know that first-hand. But that doesn't mean it isn't just as good for staying in shape. Maybe it's better because you can get aerobic fitness without pounding your legs. I don't do that many long rides. My daily ride is 15-20 miles, but over rolling hills. Sometimes I'll go on a local rails-to-trails that is 31 miles, but flat. I'm breathing a lot harder after 15 miles on hills than I am after 31 flat miles. I'd like to do more longer rides, but they really take up a lot of time.

big john

No doubt running is great aerobic conditioning and you can get a lot in a short time. Also, safer and cheaper if you can keep it up for years. A lot of people come over to cycling when running injuries get in their way. Some of the ex-runners I have known start cycling with a huge aerobic base and they're usually lean and understand their bodies. Unfortunately a couple of them dove into the deep end before they had a chance to learn group riding skills and bike handling which can lead to crashes.