I did all my long runs off road and suffered no ill effects, in fact I thought it did me good as the rough surface and hills helped to develop muscles that roads don't.
Don't worry by 18 miles on a marathon everthing hurts whether you've trained on roads or not.
I also attache it into the inside of my shorts/tights and place it just above the hip, where theres less bounce. I aslo noticed skipping can happen when the batteries are low and I read somewhere that the antiskipping device will only run for a certain period of excessive bumping, I may have dreamt that though.
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Deneo
