I find the longer slower runs help 'blow the cobwebs away'. Although I know the benefits of the faster sessions, I feel better if I go out further. Lately having got fed up with some routes, I now drive a few miles, dump the car then run a new circuit, and it feels great.
I trained for a marathon on a treadmill.
I had to do 2 sessions of 90 mins to do a 3 hour run.
From memory I think I watched 2 6 Nations games whilst running so time flew by!
Yes, give it a go, sad to use the treadmill too much. Many of my routes are really muddy now, i know some people like that, but some are so 'sticky' that I can't actually run.
I just drive somewhere different, with somewhere 'safe' to leave the car, or get someone to drop you off somewhere (not 20 miles away though!) and then get a buzz from a total change of scenery, it works wonders!
I agree that treadmill running can be more tiring as I tend to go at my normal steady pace or faster whereas when running outside I will slow down or even stop every now and then.