my fitness

i'm 21yrs, 6'4",200pds, look very atheletic but l have a severe fitness issue.
l got of skills and love playing sports but the problem is anytime l'm normally agressive l get tired within 5 mins, it shocks everyone and looks rediculous sometimes.
lt like there would be this burning in my body. produce soo much heat, l'll be sweating and it feels like l dont have any energy left. lf l try to go on, it looks like lm about to faint.
the other part to is that, l can do non- agressive things for longer periods than most people could.like walking in a marathon or walking for 8hrs straight in a fastfood res. without taking a break.
lt really been buggingg me so if anyone has any suggestions l'll be glad to hear them.

[barn babe]

I'm thinking blood sugar issues of some kind, but I'm not a doctor. Have you had a complete physical?  It might be a good idea to get a physical exam done, just to establish a baseline.  

You might want to start a higher-intensity workout program. Nothing too severe. Do you have access to a treadmill?  Try using a treadmill and vary the rate of exertion/incline, so that, say, you walk for 3 minutes, then up the exertion/incline rate and do the higher rate for 30 seconds to 1 minute, then drop it back down and walk again for 3 minutes, and so on. See if you can do that for 15 minutes or so.  Then start upping the time you are on the 'mill to 30 minutes total.  This could help you build your capacity for exertional activity away from the gym.  

I'd start with a complete physical though, and go from there. Keep us posted.