I am SORRY for being a jerk.

However, I have to say, it is NATURAL to feel your legs burning after 10 minutes if you are out of shape. If you have never felt your legs burn, you have not experienced life or pain. PAIN IS GAIN, buddy. If you wanna build stamina, first make yourself burn after a longer time. I have no idea how fast you're riding, but 10 minutes at a fast speed on a mountain bike will make anyone's legs burn. If they don't burn too bad, keep going. Let the acid break your muscles down so they'll grow back stronger.

Toughen up, buttercup.