As a postscript for today's 'experiments', I would like to add that the 13 mile ride I did left me feeling the same (knackered) way as I do for a qualifying ride in the 50km challenge. I appreciate the physiological reasons why this should be the case, but what I have not factored in (as a Newbie to Fixed Gear) is the powerful dopamine hit from exercising harder. It will come as no surprise to you Fixed Gear aficionados that about an hour-and-an-half afterwards I was considering going out again.