I am a semi-avid biker. I often do 75+ miles a day, sometimes over 100 if it's truly a free day, and a few days I went over 150. I can bike for hours and never stop. To add to that, I am a very hard, aggressive biker. I do what bikers tell me not to do- accelerate hard and fast, put full force into going up hills, pedal so hard my front tire is off the ground for over a mile (happens to me a lot), jump from a stop and get to top speed as quickly as possible, etc. I can do that for hours. But when it comes to running, 1000 feet is a great achievement. I get exhausted. Why is this?