I've been using a pedometer app on my phone. But the heartbeat option is something you need to get the most calorie-burning. I've actually lost weight since the pandemic started, about 30 lbs. I think it's in large part of not going out to eat much anymore. In the beginning, it was because I couldn't, then I got just used to not doing it. But I have been walking a lot more, too. Transit isn't as handy anymore - it doesn't deliver me right to where I need to go so I have to walk.
Anyway, I would expect that the Pi 400 would perform well. I have a Pi 3 and it is very usable. But I haven't compared it to this laptop. Know any good benchmarking programs for Linux?