For me it usually happens later than that. 45min sound a bit short. Based on
my own experience, there can be a few possibilities:
1) You increased your distance/time too quickly and your legs can't take it
2) If it's really hot and humid, you could lose a lot of sodium & potassium,
which would cause cramps for sure. It could be that you don't hydrate well/get
enough electrolytes in before the run or as part or your overall diet.
3) Could it be your running form? Some say you got to relax on the toe-off.
(when I use my toes hard in the last 1-2K of a race, I get more cramps than I
I think there are other potential causes, but they escape me right now...