I've been off my bike for about two months, spent my summer traveling. Now that I'm back on it, I'm finding myself having to get up and crank slowly to go up hills, and brake to slow down. It's geared 42x15, which treated me quite well before the hiatus. If I'm putting around 15-30 miles a day (commuting) on the bike, is it worth buying a 17t or something cog to get back in practice, or will I be back in form soon enough to just put up with it?

Cheers,

I wouldn't bother. It won't take long to get it back.