I'm a 40 year old guy who's been sitting behind a desk writing software for two decades. I was in good shape in my 20s but haven't exercised significantly in a very long time.

It took me about three months to get up to that speed, cycling every other day. I started out doing 10 miles, at 10mph.

Three months isn't all that long for an exercise program.

Color me skeptical. In order to burn 1000 kcal in 60 minutes a cyclist would have to sustain a power output of about 280 W. Do you have a calibrated power meter, or are you going by the "vanity" calories displayed by most stationary bikes?

There are certainly plenty of trained cyclists who can maintain that power output, but I've never seen anyone reach that level after only 3 months. If you really did that then you're an extreme genetic outlier with an unusually fast training response.

I don't know, actually. I'll check it out later today. I do know my heart rate is around 150-160 throughout.

I did a lot of cycling in my 20s and while I'm at least a decade out of shape I'm sure I'm relying a lot on past training.

High aerobic fitness evaporates pretty quickly. The training you did a decade ago will not carry over to today. However, there are a few genetic outliers who can quickly build up a high level of fitness with just a short training program. You could be one of them, but it's more likely you're just getting bad data.

Absolute heart rate doesn't tell us much about calorie consumption because there's a huge variation between individuals. If we know the athlete's size, sex, and threshold heart rate then we can start to make some kind of estimate. But to get an accurate number it really takes a properly calibrated power meter (or more complex lab tests involving exhaled gas measurement).

It seems doubtful the measurements would be far off. The numbers fit with the expected 50 or so calories per mile biked.

It's doubtful that your measurements are even close to accurate. If you're on a stationary bike then the distance displayed is completely meaningless. The equipment manufacturers just show those numbers to make their customers feel good. In terms of calories they are in no way comparable to real outdoor cycling. You can get a great workout on a stationary bike but you need to be realistic about the true power output.

What is the actual power output in watts? Has it been properly calibrated? If not then you're just fooling yourself.

"Completely meaningless" and "in no way comparable?"

These statements are rather extreme and seem impossible to support. Energy output per mile with real world cycling has enormous variance as well. Why so much concern for precision regarding one and not the other?

I don't think there's much point in continuing here.

I'll continue with one last comment just for the sake of others who might be reading. The "distance" displayed on most stationary bikes is calculated by counting flywheel revolutions and multiplying by some arbitrary factor. It has no real correlation with the actual distance you would cover by pedaling a real bike with the same effort. It's a totally pointless number that tells you nothing useful about caloric expenditure, but some customers want to see it so the manufacturers put it on the display.

You can get an accurate calculation of real world cycling calories with a power meter. Everyone's cycling efficiency in terms of power output per calorie burned falls within a fairly narrow range so converting from time and watts to calories is simple. A good power meter accurate to within a couple percent can be added to any bike for about $700.

Smart trainers like a Tacx Neo or Wahoo Kickr also have accurate power meters built in. Those are substantially different from typical stationary bikes.

fwiw, I checked and the bike does have a power meter, I think it's an RS3 Lifecycle (or some similar model). I do a sustained 220-250w for an hour.

I understand the translation between power and mph is somewhat approximated, but I think you could be more generous. It's not unreasonable to make this kind of approximation and the numbers aren't meaningless.

Not that it matters, but reporting mph for a stationary bike (indoor trainer I assume) is meaningless in this context. This could mean that you're cycling anywhere between 0 and 2000 Watts.

I cycle on and off for ~8 years (35yo) and going at 20mph in the real world for an hour or more became possible only after a few years for me.

same here, I'm in my late 20s and was generally unhealthy because of the pandemic, and I burn 630-700 calories per 30 minutes worked out

That is unlikely. How are you measuring calorie burn? You're probably overestimating.

As a point of comparison, an average size man would have to run at a speed of at least 11 mph (18 km/h) in order to burn that many calories in 30 minutes. Not impossible, but well beyond what most of us can manage.