Re: Had one of them runs tonight...
Like already said treadmills are easier, but they're also a lot better for your knees. Maybe step up the pace on the treadmill a bit as some sort of compromise?
To be honest though, when training for the great north run last year, I did a lot of treadmill work to start with, then when I knew I could run 10k I got out on the roads. Now I never run if it's not out on the road