When you run outside, slow down - then slow down some more.  See how that feels. 

I started running on a treadmill and wasn't too sure about running outside when I finally decided I needed to go outdoors to prepare for a race.  The hardest thing I had to learn was setting a comfortable pace.  I always started out too fast and got winded.  I blamed it on the hills, but truth was, I was just trying to go faster than my body could sustain without the treadmill belt doing some of the work for me.  When I was able to force myself to really slow down, I found I could run the distance, whatever that was.