All depends on what your goals are. A guy cruising around with a 34 tooth cog on his cassette who is asking questions like this on an Internet forum is most likely not someone adept at training who has years of base. I just don't want anyone to think that 2x20s or sprint intervals and the word "fun" belong in the same sentence. There are training videos that allow you to do things like this. They're called "Sufferfest", not Funfest. also, most guys who are racing, even at the cat5 level are putting in 10+ hours a week when the weather is good.