OK. Between 10 and 15 hours per week is pretty close to ideal, in my opinion. If you're still going to run, adjust the time spent on the bike downwards. So, for the sake of argument, say 15 hours a week if you're not running, but ten hours a week if you're running two or three hours.

Sounds as if you're a bit overweight but pretty aerobically fit. Being in Colorado won't make any difference to your training because you're accustomed to the altitude. But if you ever go and compete at sea level you'll have an advantage, of course!

The big changes from running to cycling are, first, that cycling is non weight-bearing. So you can spend much longer riding than running because it doesn't beat you up as much. And second, the way the leg muscles are used is very different. Because your cardiovascular system is in good shape, that means that your legs rather than your heart and lungs will be the limiter to start with. That being so, a good strategy is to use a slightly lower gear than feels natural for a given speed, and pedal faster. That is harder on the heart and lungs but easier on the legs. Typically, fit and experienced cyclists will pedal at a higher cadence (>90 rpm) than beginners, because their CV system can take it and it minimises muscular fatigue.

Without a HRM you're going to have to go on perceived effort. Broadly speaking, since you have upwards of 10 hours per week to train I'd spend about 70 percent of your time at cruising speed. That will build more aerobic fitness and build you a good base. Throw in a couple of sessions of maybe an hour each at a more strenuous, but not brutal, pace - the sort of effort that allows you to talk, but not in complete sentences. Then a couple of times a week do some high-intensity intervals. Thirty seconds going as hard as you can, thirty sessions rest, repeated ten times, is a good one. Don't make all your rides the same length. A couple at three hours, plus four shorter ones (some intense, some very easy) might be a good mix.

These sessions don't have to be strictly separated. Being in Colorado you'll have plenty of opportunities to ride hills. A long ride that is mostly at cruising speeds but includes some hard efforts on the hills is a good way of mixing it up and staying interested. But plan your rides so you have time for recovery. Just like running, putting in hard efforts day after day is a recipe for exhaustion, not progress. That doesn't mean you have to spend many days off the bike altogether, just that some days (usually the ones following your hardest sessions) need to be done at a very easy pace - so that it feels ridiculously easy.

None of this is rocket science. There are plenty of books on cycle training, but most will focus on either HR training or powermeters. You might not want to make that investment until you're convinced you're sticking with it.

One other thing. When you buy your road bike, get some advice on making it fit you properly. Good bike shops will do this, and it makes a massive difference to your comfort and efficiency.