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I use the treadmill a lot, it fits in with my week, and as I don't like running alone in the dark it's a god send. I always try to get out into the fresh air if I can, but running on a treadmill is better than not running at all I think
Absolutely Jo, better to do a treadmill session as a substitute than none at all. Even if you add a 1% gradient, you'll still find the transition to outside running a bit of a challenge to start with as way your stride works is appreciably different - the ground doesn't drag your striking foot back when you run outside!
But think of the cardiovascular benefits, which are obviously just as good as what you can get outside, so long as you do similarly structured sessions.