Assuming your 37 mile ride was solo and your half century will be a group ride, the half century will probably be easier since you'll be in a pack and can draft half the time. If you're bonking then just ride more, you're out of shape. You're pumping a lot of Gu and stuff into your body (and spending a lot of money on it) for short rides. One every 30-45 minutes would give me heartburn. Don't know about that, I hardly use anything. One Cliff bar is good for me on a metric century.