I've found the only problem with treadmills is they tend to be deceiving. I was working out pretty regularly at the Y last winter, and ran anywhere from 30 minutes to 80 minutes on the treadmill. I kept trying to increase my run by one mile at the end of the week. Come April I went out to the Monon and could barely run a mile and a half. So I'd be careful to at least mix in the pavement and would probably recommend it for your long run during the week.