I started my running doing late nights on the treadmill after getting inspired by Runner's World Half-Marathon Challenge. I set myself goal paces for certain amounts of time and I went from DNF to slowly accomplishing each workout. The treadmill was integral to my training because I used it like free weights - the speed was my weight the time my set. I knew how much improvement I was making by setting the weight/speed higher and and doing more sets (read: running for a longer time).
Running outside (on my own) can do the same thing, it keeps you honest - but honesty is not always what you need. Sometimes you need that prop, that assist, that helps you trying harder than you normally would. Sometimes you need to run on the back of the treadmill or cheat on a rep.
However, you always need to translate this training to the field, out of the weight room. That will be shown a week from now on the hills of Frederick, MD.
Training note: Sunday, 8 miles, 66 minutes