Originally Posted by hooligan
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.
To a point, but pushing like that can also get you injured in a few ways, can pull a muscles, which if not detected can become torn muscles, which has the slight possibility of taking some bone with it. Ligaments can tear. If your legs are tired, it's also likely that more of you is tired as well, for example arms and shoulders, losing grip of the bars when cornering is a very painful experience when your arms are tired, even your head is most likely tired as well which can lead to some potentially bad decisions. If your body is not into it that day, don't go out, it's in your body's best interests to rest up.
The best thing to do is to listen to your body and not what someone tells you to do. Feeling the pain when starting out is never wise mid to late ride is fine, but from the start(the first few pedal strokes) you can hurt yourself.
For me, I have the fixed gear for when the legs are tired from using the mtb, it allows me to spin lightly without having to worry about working too hard. Always feel good the morning after riding the fixed gear. Or I lift weights, focusing primarily on the upper body.