You sound a bit like where I was 6 months ago, though probably less fat and unfit, and much less time off the bike. My previous riding was almost all road and I’d never owned a mountain bike.
I talked to the local LBS who gave me what I think was really good advice, spending a lot of time finding out what sort of riding I fancied- cross country (and I am lucky in that I can do it straight from home). We settled on a 29er hardtail and the guy put some effort into talking me downscale to a less expensive bike (Whyte 529), to see how I got on.
I’m really enjoying it, apart from Friday’s rather good crash on to gritstone boulders, which has fortunately left me with only flesh wounds.
I think the 529 was about right for where I was at the time, not so heavy and low spec that riding was no fun, but not silly expensive or fancy. Sadly I had a go on a carbon hardtail a few weeks ago and the Whyte 29C-S is now firmly on the shopping list.
I like the 29er for covering ground and I also feel comfortable on them. Other here will know far more than I do about the choices available to you (though Whyte’s 629 is £ 999) but I’m very much a happy camper at the moment.