I've been off my bikes for 4 months straight and I decided to take the fatty on the Jan 1 ride because it was short. 10 miles. Yesterdays coffee ride was 28 rt and The Moose was already set up so I took it again. Even pumped up to 18 psi, it was a workout. I totally got my ass kicked by the roadies and commuters. But mostly it's because I'm out of shape. I did the same run last year and had no problems keeping up. I hope to have a set of street tires and a set of studdeds so I'm set for whatever season. But it won't happen this year.
You didn't say what f/b you have.