Nice! If you want a more quantitative way of comparing performance, a power meter is a great way. Average speed is affected by the route, the weather, etc., whereas power is much less influenced by these things (not entirely of course --- I put out more power on hills, and temperature has an effect, etc.).
The only downside is they are really expensive, and for MTB I'm not sure what the options are.
For reference, ~800kcal in ~2hours should mean roughly 110-120W average power, but without a power meter that's pretty much just a guess.
Cadence definitely the right place to start, since it shouldn't break the bank.
I'd do some research on cadence if you haven't already --- you mentioned getting up hills in a higher gear to be faster while also keeping HR down. That's definitely an option, but it can come at the expense of your legs (and worst case, knees).
Conventional wisdom today (as I understand) is that if you thrash your cardio, it'll recover fast; thrash your legs, they'll recover more slowly. So there tends to be a bias towards higher cadence (80-100rpm or so) for performance, with lower cadence useful for strength training.
Good luck! Cycling is a wonderful sport. Focus on the numbers and gadgets if you like, but at the end of the day remember to have fun :)