I am not sure of my calorie consumption as I am just getting into +200km rides, but I will be learning more as I start covering longer distances. I've got a 50mm front and an 80mm rear now that would work. I have changed the bearings in them a couple of times, and usually get about 10,000-12,000km out of a set, but even faster wheels might find their way onto the V20. The bad thing about having different depth wheels is the stem length on the spare tubes. Too long for the 50mm and drag increases. Too short for the 80mm and you are screwed. At first, flats on the front wheel had me nervous because I didn't know how to get the wheel back on the bike efficiently. But that is not an issue now.
When you get a power meter, you will be able to very accurately know how much energy you put out over the course of a ride.
For basic guidance, Coach John Hughes has some pretty good books and online papers. If you want to train without a powermeter, I think his pacing guidelines are very good. For instance, he describes digestion pace. As we all know, you can't eat when going full throttle but what is the pace that you can ride and digest fairly well.
Some racers have tried to model what percentage of FTP you should start at and then at what rate it bleeds down to a relatively low value that you can sustain for a long, long time. The cause of this fatigue isn't entirely known. For top racers, they might start a 24H race at 70% of FTP and finish at 50-55% of FTP. Of course, someone like Chrisoph Strasser has an FTP of 420, so looking at his online Strave files, it is easy to think he is riding very hard at 290-295 watts but that is an easy level for him. Let's say someone's FTP is 250 watts, they should probably keep it to maybe 175 watts in a very long event but will be down to more like 115 watts as things progress. It is important to accept but also not push too hard, too early when it is very easy to do so. There is a very bright engineer (Dave Brillhart) who won the 444 mile Natchez Trace race, he wrote an AP for the Garmin that models this power and also things like when to drink and eat with popups. I am not saying anyone needs such a tool, but learning one's limits is very important and just takes experience. Randonneuring is probably the easiest and most accessible venue to do that.