push yourself then. you know your limits: see if you can get faster in your max gear, or see how far you get in your max gear. keep a record, see if you can break that record next time. as someone else around herre is fond of saying,
"Pain is only weakness leaving the body!!"

sorry, we appear to have forgotten that you're a MTBer carrying around about 5 useless kg of steel and whatever! try standing and pushing for a while: a change in posture, especially when you're tired, can do wonders as you use your muscles differently. of course, we all know that standing to cycle is not efficient, but it is possible for short distances and it makes you strong.