I think that depends upon the person. I think it is harder to go outside and run, but I have a friend that runs a lot and things it is harder for him to run on a treadmill. Not because of the inside vs. outside thing, but just harder for him in fitness.
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2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun