It's not surprising that you are gaining weight. Your BMR is around 1600. If you're eating 2500 a day that means you need to be burning an average of 800 calories per day on the bike plus other activity just to maintain your weight. 20 miles a day on the bike at 14mph is likely to be burning you about 500 calories, maybe less, I would guess this is just simple over-eating, or over-estimating your calories burned on the bike.