Quote Originally Posted by 246marathon View Post
I sometimes don't feel like my heart rate gets very high (may be because of all the running)
I think you're right - for one thing, any given person's MHR for cycling will be around 5 bpm lower than their MHR for running - and also to me, running is just way more aerobically demanding. I'm running much more now than I ever did, and one thing I found this summer is that I never, not once, ran out of air on the bike. I don't race or push myself terribly hard on the bike, but last year there were definitely hills where I was struggling for breath, and sprints where oxygen was as much a limiting factor as muscle. Since I started running more, that's just not the case.

Still, you can get your HR pretty high on the bike. IME I just have to work at it a lot harder than I do running.