Treadmill Run = Awful

I used to enjoy running on the treadmill. Now I can't stand it. After work today I decided to do my 3 mile run on the treadmill. That was a big mistake. It was awful. It was even painful. My shins started to hurt and I almost gave up. I finished my 3 miles by doing run/walk intervals. Which is fine, but when I can run 7 miles without stopping to walk outside I should be able to run 3 easily on the treadmill. I don't know what has shifted, but now I would much rather complete my runs outside. Plus, according to my Garmin my pace is faster when I run outside. Hmmm...who knew!