in all likelyhood it is a matter of conditioning. If you do the miles you should be getting stronger with more endurance. Perhaps you are riding too hard and dipping into anaerobic sources too much. That will have an impact on your running out of gas condition. The condition of running out of gas at 55 minutes should not be a permanent condition. You definately will get better if you train right. I suggest that you "being alright" after a rest and a snack probably has more to do with the rest than the snack, at least at 55 minutes.