I do my running at about five in the morning so it's still quite cool, this mornign ist was only twenty oen degrees, quite comfortable really. But regarding the topic, I agree with Rabby, while you do need to make sure you prepare (getting dehydrated isn't a good idea). I'd have thought that even if you do find the running harder, as long as you put maximum effort in you should still get similar results.
And I do get a fair splodge of stinging on the face where I've shaved, probably due to salts being deposited in the broken skin. In any case I very much doubt it's a problem

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