If you can't train with power, the simplest approach is either to use the lowest, or use a simple standard number per mile of your own.
For example, I generally use 10 calories/minute or 40 calories per mile (there is really a k in front of those, yes) since on mixed terrain rides I'll usually average between 15 - 17 miles an hour. That is lower than what most fitness software (let alone gym equipment!) will report but feels closer to reality to me over the years - and I'd rather err low than high.
On Zwift rides on a smart trainer where it gets both heart rate and power readings and knows my weight but also the speeds tend to be inflated, the 40 calories per mile seems to be pretty much in the middle of what Zwift calculates - too high on rides I do at lower average power, too low on rides I do at higher average power levels.