How tall are you?

I always gain weight after a long ride, but it's water, so it's gone within a day or so.

My trainer told me to forget about the "fat burning zone" for cardio workouts. Interval training that gets you up to a higher heart rate is better. A lower percentage of the calories you burn might be fat, but you'll burn more calories overall, and as a result will burn a larger total number of fat calories.