I used to have a 12 mile commute that took nearly an hour - It's almost always cool in the morning, and I'd ride slow for the last mile or so to minimize sweating, then change into my regular clothes at the office. No shower or wipedown required. On the way home I'd typically just wear my regular clothes.

I don't know how you're ruining your pants, but if it's bike grease or getting your pant legs sucked into the chain, a simple ankle strap will let you avoid that. By using a drive wax lube (I use White Lightning, but there are others), that also minimizes the dirt transfer if your pant leg does brush against the chain or chainring.