I've had this since I was a kid: I can't seem to run or exercise for too long until I get very tired (i'm not overweight, in fact, I'm quite slim)?

Tired with exercise. There can be many causes to give this symptoms and i will suggest you see your doctor if you have not done it already and find out what is causing it.You may be anemic, there may be ptoblem with lungs and your heart needs to be checked as well, and we need detailed family and past history.Blood test, ekg, pulmonary function test etc.