Also, if you've got less than 50 miles in the saddle, then you really need to work up to longer distances. If you asked me, and I know that you didn't, what I'd advise you is that it's probably more important that you work up your cardiovascular endurance and cycling legs than it is to ensure that you're burning the most body fat during your rides.
What this means is that you probably want to do some 10-15 mile rides for at least a couple of weeks before you push out your distance even further. Nobody starts their running career by going out and doing a marathon or even half marathon. You need to build up your cardiovascular base endurance and get some cycling legs under you before you should start tinkering with low energy state fat-burning training. There's lots of stuff out there to read on this, and I don't know who are the real experts one should be reading and who's just talking out of their ass (hint: I'm probably one of the ones talking out of my ass, since what I say is just based on a smattering of reading and my own personal experiences, not some kind of scientific training in the discipline).
Here's an example of an article I felt was useful to me in terms of heart rate and building up the cardiovascular base. Using this guy's formula and rounding up or down 1 or 2 bpm I arrived at 130 bpm as my target heart rate for my cardio base-building rides. On most of my solo rides this is what I stick to for now. Sometimes I set myself a time challenge on a particular route to see where I'm at (set some Strava PRs/KOM attempt on some segment or other), and when I go on a group ride this heart rate target goes straight out the window. But for solo daily mileage rides the 130bpm target heart rate is what I shoot for. It'll be different for everyone since it depends on age and some sort of assessment of one's physical state.
I'm doing lots of miles up to my 32.5 mile route on an empty stomach and at this target heart rate. I would not advise you to do this. Go for those 10-15 mile rides and just build up that cardio base for a few weeks. Then push it out to 20 miles, then further, as you get your legs under you, your cardiovascular system gets stronger, etc. And eat something before you ride. Now's not the time to be doing empty stomach rides, since you've barely even ridden at all. This is way too early for that.
If you don't have a heart rate monitor I'd suggest that it's a very good investment to make. I was on a group ride this past Saturday and we got to a hill climb section where I got dropped from the main group of skinny people, and was pushing really hard to keep up with this group of two riders. I was staying with them, but I looked down and was pushing 155-160 bpm heart rate. I knew immediately that I would not be able to sustain that over the few miles that this hilly section still had in front of us, so I dropped myself from that group, got my heart rate down to 145-150 bpm and just kept my own pace through those hills. This was based on experience, and an understanding of where my body was at. The 145-150 heart rate was at the threshold of what I could sustain over a long distance, and the 155-160 bpm was over that threshold, which means I could know immediately that my ability to sustain that was not going to last as many miles as we still had ahead of us.
Anyhow, keep it up. Get those miles in, build your cardio base. Work on your techniques. Learn to cycle in more of a 90-100 rpm cadence rather than, say, 60-80 rpm, and you'll probably find, like most folks do, that you can keep that up much more easily by shifting lots of the work to your cardiovascular system, rather than mashing really hard with your leg muscles and blowing them up. It feels really wierd pedaling at 95 or 100 rpm at first, but it's something to work up to, and after you get to where you can do it, it pays dividends bigly, to paraphrase our lard-ass in chief.