I think I can add an interesting perspective. In college (5 years ago) I was an avid runner. I ran 3 miles every day, and about twice a week I went 5 mi. in 30 min. I love running but have recently discovered it's very hard on my knees. I took up cycling with a litte running mixed in. I think running is a far more consistent work out, and it can be very challenging. But I have enjoyed biking as much or more because I can go farther, see more, and actually enjoy an equal workout if I simply take the time to go 10 or so miles. Plus, if you've ever come up to a hill after cycling 8 miles, there's nothing in running to equal that dread. That said, if I only have 30 minutes, I run because it's more strenuous.