Tom- I was in the same boat as you about 3 years ago. The biggest difficulties are: judging pace outdoors- if you have a iphone, or other gps-type system, this is easier, otherwise, pick a route you know the length of(measue it on map-my -run if unsure), then work out how long that would take at your current treadmill pace, and make sure you set off slower than that.
To make your treadmill runs feel closer to the same effort - put 1 degree incline on the tready.
This sounds silly, but thing like going round corners, steping over kerbs, avoiding bollards all take time and energy- well worth running outside to get used to this.