Even the skinniest rider has plenty of body fat for fuel. At 6' and 143 lbs I hear "you need to eat a cheeseburger" fairly often. It's kind of rude, but I realize it's from envy. Of course the riders envying me are usually much stronger into headwinds, but they're not thinking of that.

Day after day of medium intensity isn't a good way to train. Read up on "base training". SST is a kind of interval, done at ~90% of threshold (the power you can hold for an hour).