Hi
Ive got a friend who had the same problem every time we did a merida or endurance event
He is fitter than me , carries far less weight , jogged etc
I never cramped up ,even went totally spent , but he suffered terribly usually after around the 3 or 4 hr mark
it was becoming so much of a problem he strongly considered not doing the events
After lots of trial and error he found that for him a concoction of
salt tablets before and during
Tonic water before and during
Camelback mixture of high 5 , tonic water ,
Sounds alot of hassle i no but it works for him , never craps now