I don’t ride bikes for fitness, I ride purely for run and any fitness gained is a bonus.
The one thing I would say is that over 4 years of riding EMTB’s at least twice a week, I have put on upper body and core muscle from having to hustle about a heavier bike.
If I start to get knackered on a long ride it’s my back that goes first from the weight of the bike - on my mountain normal bikes after 4-5 hours my legs will start to feel it, but on the ebike it’s my upper body strength that’s found wanting.
I do zero other “fitness” activities and have never been down a gym since school, but do play the odd bit of tennis and cricket.
I use a Garmin watch with my bike, as I can connect to the steps sytem where I have removed the display as I hate any clutter on my bars, and use the watch to get battery info etc.
I don’t really look at the fitness data the watch gathers, but I have noticed is that there is very little difference in the effort put in on my ebike and a normal bike.
Most of my riding is solo, and I go out for 2-3 hours, but the bike in trail mode, and go full tilt boogie without stopping.
I think if you are into riding bikes, you will go full tilt and push yourself no matter what the machine you are on.
I think it’s a misconception many people have that getting an EMTB means you can be lazy - not true, most people I know including myself still go at climbs etc hammer and tongs, your just doing it faster.
The beauty for me of the EMTB is you can get more riding in.
I still think a normal bike is more fun once the trail is pointing down, but what’s the most fun is being able to do way more laps etc, and that’s why my EMTB will be my main ride for the foreseeable future.