Wrong board, should be on the training board.
Anyway... Possible reasons are that you are out of shape, trying to do too much too fast. If you havent trained in a year pretend you're starting from the beginning again.
Could be you're cramping because you're dehydrated, therefore drink more water.
Could also be cramping because you are aerobically unfit, in this case you need to do some aerobic training a few times a week.
