I have never really done any cross training during my two years of running. But over the last weekend I happened upon an old stationary cycle and I tried it out a bit today . Now - when running, I only have to run slowly for a few minutes and my HR will go up to around 115. So I started pedaling this machine - I think I had it set about "medium" - it had a fair bit of resistance - so I started pedaling away - my HR rose ever so SLOWLY to 106 -- by which time the tops of my legs were BURNING!!! Conscious of the fact that I have to run a HM this coming weekend, I didn't do it for long -- but frankly I don't think I could have for very long anyway ..