How big of a change is it running on the treadmill to outside?

For the most benefit to a beginner, run a half hour, taking walking breaks as necessary, but keep going for a half hour. Going "all out" is never a good idea, that is how you burn out. Save it for a race if you are going to run in any. A study at the University of Utah showed that outside runners ran 4 minutes faster in 5k races than treadmill runners.