I ran from 7th grade through college with 5k around 15:00 and sub 4 1500
I now MTB exclusively. I do a little gravel and cx racing but MTB is where it's at.
I ride about 500 hours a year. been doing this for about 7-8 years straight. I climbed about 500k feet this year. Not bragging just establishing that I ride more than, like, 20 times a year.
You should get a quality 29'er hardtail with good components, fork, and wheels. I'd go used unless you can spend on new.
A hardy bike built for throwing around will be closer to 28 lbs whereas you can get an XC race whip closer to 20-23 easier. The party bike is a lot more fun in non-race scenarios but leaning how to rip on an XC bike is awesome too.
Look at specialized epic, fuse, Kona honzo, trek stache, surly krampus, etc. Either a trail or xc 29er that's less than....5 years old. Geometry has changed quite a bit since then - for the better. I wouldn't buy a MTB that's steeper than 69-70 degrees up front.
29er hardtail. Don't get a full suspension bike. Don't bother with 27.5
I repeat don't get a full suspension bike. If you really need convincing reply here and I'll explain.
MTB is far superior to running. You can absolutely thrash your legs, core, arms, cardio. And it's fun as f---. I've gotten really good at technical climbing, clearing gap jumps, ripping downhill trails, and so on. It's super rewarding to ride with skill and pump through a trail.
Running doesn't have anything comparable to heading out for a 6 hour back country adventure on singletrack with your bike. It just doesn't.