I think this is pretty common. Running on the treadmill seems easier because there is little in the way of resistance. You are basically jogging in place and the movement of the tread belt is propelling you rather than your muscles. When you are outside, you have to cope with wind resistance, terrain changes, speed variability and all of your forward momentum comes from your own body rather than the treadmill's motor.
I would suggest cutting back a bit when you run outside and work up to 30 minutes. So, maybe start out running 20 minutes outside, adding a few minutes until you get back up to 30. Running on your treadmill should not interfere with your outdoor running, but you may find that you become bored with it after you've gotten used to being outside....
ETA: You may also need to start out at a slower speed when you are outside. It can be more difficult to regulate how fast you are running if you are used to the treadmill doing it for you.
Edited by: LITTLETEALOVER at: 4/26/2011 (13:07)
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