i'm probably displaying a complete lack of ignorance of physiology and training here, but what is the problem with your body burning carbs/glycogen in preference to fat whilst riding?
i know that you only have much less energy in the form of glycogen in your muscles compared to the body's store of fat and that if you 'teach' your body via training to somehow prefer to burn fat/spare your glycogen you will have more glygogen left when you call on to use it for the higher end stuff but......
if, say, you pretty much deplete your glycogen stores, why can't you just keep going by ingesting high simple carbs in sports drinks and gels etc.? won't your body get all the carbs it needs through that ingestion and therefore your buring carbs in preference to fat for fuel is not really an issue save for on super long (i.e. 6-7 hr + rides) rides?
Low intensity, long intervals (longer than 45 minutes) uses up more fat stores than glycogen stores.
High intensity, short intervals (less the 20 minutes) uses up more glycogen stores than fat. The high intensity will increase the metabolic rate.
You can only ingest so many gels ans sports drinks before the body stops processing it. Therefore it is best to top off the glycogen stores a day or two before the big long ride.
Also not many of us do long rides. Most people ride 2-3 hrs max. The adaptation to long rides comes through training and fitness. Part of training is letting the body adapt to stress and striking a balance between calories taken before the ride, calories taken during the ride and calories replenished after the ride.