I get the high only from strenuous exercise that lasts about an hour or more, like soccer for example. Half hour runs or weight lifting do not have such an effect, and I don't find the reward worth the pain in those activities which means I do them in a more reasonable pace.

This suggests you might have to reach a certain level of fitness to be able to strain yourself enough to get the high and this level varies between activities and people. There are activities like swimming that don't give me the high at all no matter how hard I try, but oddly enough swimming is my favorite form of exercise.