Not races per se, but I did once snort a scale model of the M25, including the Dartford Bridge, in one breath. More than half a gram, I recall, which really should have hospitalised me.
I was around that period that I had an extremely embarrassing visit to the dentist when he kept injecting more and more novocaine into my gum and couldn't understand why it wasn't working. In the end I had to own up to the fact that I 'probably have quite a high tolerance to novocaine, if you see what I mean.'
He actually found it quite amusing.
My God I used to do some heroic quantities.