Why can't I do more then a two hours of cardio a day.

You can. However, you need to eat every 2 to 3 hours if you are going to do a long, multi-hour hike or other major endurance cardiovascular workout. If the body becomes overly exhausted, it will start saving food as fat as a survival response. However, this doesn’t usually happen if you do this infrequently or if you nourish yourself properly.