So you did 62 miles in about 4 hours, which is an average of 15.5 mph. My opinion is that, if you can do that, then you could keep on going and do 75 miles the first day and then 75 miles the second day. From a physiological perspective, you are burning mostly fat for energy at an easy pace, but you will still will burn some glycogen (muscle sugar). Your stores of muscle glycogen will vary depending on your training. Once your muscle glycogen stores are depleted, you will experience a noticeable drop in your power output - it will quickly feel more difficult. You can't conserve your muscle glycogen per se, but you can prevent using it up unnecessarily. That means you should not do extreme bouts of power output like sprinting or climbing too fast - always stay aerobic. If you do deplete your glycogen stores, you can continue on indefinitely, it will just feel more difficult than before you depleted your glycogen stores.
Over the years, I have experimented with long distance quite a bit, both aerobically and anaerobically. And we had a PhD biochemist on our racing team that I used as a reference after studying exercise physiology on my own. I have been able to go out on a DF and easily do 64 miles without eating or drinking anything. I can go out now on my V and do 100 miles easily, not even feeling very tired afterwards. Here are some recommendations. Eat a few hundred calories and drink (but not too much) a little before you start. During the ride, drink a mixture of water and glucose (aka Dextrose, which does not have to digest and goes straight into the bloodstream - Dextrose is cheap in bulk) as soon as you experience any hunger pangs. Make sure you drink enough (but don't overdue it). Once you get dehydrated you can't make it up because it takes 24 to 48 hours to rehydrate. Staying hydrated a couple of days before you ride is also important, otherwise you are starting out at a disadvantage. For food on a ride, I prefer cliff bars. But there are many options. Something solid is good after riding for a while. Eventually, even something like a turkey sandwich or a chocolate bar might hit the spot. Another problem some people have is cramping. Dehydration will cause cramping. And so will electrolyte deficiencies. If you have a cramping problem, you can make up some electrolyte supplements from potassium, magnesium, and calcium supplements (or you can buy them ready-made for cycling, but more expensive). Sodium and chloride are also electrolytes, but people generally get enough table salt so that this is usually not a problem. If I did take electrolytes, I would either carry them for later in a long ride and maybe take them, or I would grind them up and put them in my drink mix. But I don't usually need to take electrolyte supplements anymore. Over the years I've adapted so that 100 miles is easy and I just have to make sure I drink enough, otherwise I get dehydrated (but don't know it until I get home and get on the scale). Getting dehydrated is also bad because it makes your blood turn into sludge, so it is harder for your heart to pump - which can cause problems if you are not healthy. Concerning dehydration, just remember that by the time you get thirsty, its too late, you can't make it up - so drink regularly to minimize dehydration.
So it sounds like you are ready now. And as you keep training, many more things will happen to you that are collectively referred to as your "aerobic engine". This includes glycogen stores, number and capacity of mitochondria, aerobic enzymes, vascularity, hematocrit, blood viscosity, lung alveoli, etc. Sorry, it's been a while since I studied the subject and I'm a bit rusty. There are two basic components you are training: 1) getting oxygen to the muscle, and 2) using the oxygen to produce power once you get it to the muscle. So under the hood of your aerobic engine, all these things are happening, which will eventually result in a bigger aerobic engine. Right now, you appear to be ready for the 150 mile ride over two days.
wish you) could do it again. good luck.stay safe.
