I Think I'm Reasonably fit?

OK guy's, I think I'm reasonably fit, can cycle 100 (hilly) miles, can run a (hilly) Half-Marathon and can run a 3 mile (600ft climb)Hill race. Why the hell then when I've walked a leisurely 15 miles (given reasonably hilly) with 4 other non cycling, mostly non running people have I done something to the top of my calf muscle at the back of the knee joint and can barely walk now? Only thing I can compare it to was when I've pulled a hamstring in the past. Answers on a postcard please. 
:shock: :? :x :oops:
:roll: :!:



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censored luck is probably why, over training maybe muscles were fatigued too much
get well soon, keep stretching it out gently
my riding:
http://www.youtube.com/user/rhyspect
Some of my Rides Data/maps:
http://www.trimbleoutdoors.com/Users/527337
He runs hilly half marathons (13 miles) but his body hasn't adapted to walking 15 miles?
Are you on crack?
Ha, made me laugh out loud. God is airborne.