more hills!
bekologist is right about eating right after exercising too! the hour after you workout is the key time to get your muscles some good protien (or so i've read). i will usually try to slam down a recovery drink soon after going out for a long ride. i bet there are cheaper ways to "recover" as well.
really i am partial to a recovery cheeseburger but i don't think i got any nutrition experts backing me up there...