train your technique.

We have an old man at the gym, he's 76yrs old. Without any effort he's trowing punches at our pro-fighters and they are all very carefull not to get hit by him..... his punches come down hard.... very hard. And no, he doesn't have the stamina anymore, neither does he train weights or anything. He just as a perfect technique. Using his hip and shoulder exaclty how it has to be done and BAM! without using power or anything.

You can also throw powerpunches offcourse with muscle, and yes, muscle helps, but you can't keep giving them throughout a whole fight. You'll never get the stamina for that. You will though if you have enough technique to back you up.

good luck!