Get one of these

https://www.worksmancycles.com/inb.html

and ride it up the steepest hill you can find. Standing up on the pedals is okay; getting off and pushing the bike is cheating (though carrying the bike up the hill is allowed). Oh, and you'll want a front brake; get the hub brake they offer and it'll add a couple more pounds to it.

Do that three times a day, six days a week. Soon, you'll have the legs of a plow horse.