I used to be the same too, never ran outside (and as soon as I tried it was crippled for a month with shin splints and achilles tendinitis!). I found that perseverence was the key outside and now I love it, you just can't compare treadmill with outside! Soo much more interesting!
(Although ran past a chipshop last night and breathed in just at the wrong time - thought I was going to throw up! But then in the gym I always seem to attract the man with the deodorant allergy...)
Definitely try outside, run walk if you have to. And there aren't many treadmill races so if you're going to do runs you might as well train on the same surface!
(And, sorry to disagree hippo, I'm now much faster on the treadmill than before I started running outside so it's great for an ego boost every now and again!)