So that's the exact same story for me! I started off with a road hybrid and discovered that I liked riding in the woods much more than I did on concrete. There is a lot of skill involved in mountain biking and it's fun to learn each technique: lifts, rolls, drops, jumps, rock gardens, climbing, and turning (I didn't even know there was a specific technique to turning on dirt but it's really important).
Expect to spend about three grand for a good full squish mountain bike. I have a Rocky Mountain and I would highly recommend the brand. They are one of the best values in terms of features, weight, and quality. There are even some really good direct-to-consumer brands now like YT. For mountain biking you definitely want disc brakes, a dropper post, and around 120mm-130mm of front travel with a total weight less then 28lbs. Global Mountain biking network on YouTube is a great resource as well as local riding clubs and bike shops. Good luck!
GMBN has a couple of great videos on buying a mountain bike [0][1], but I can really recommend a direct to consumer brand such as Canyon or YT — especially if you’re not sure whether you’ll commit to the sport. You get a lot more bike for your money compared to the store brands, and you’ll have an easier time getting into the sport the better your first bike is.
The major choice to make, irrespective of brand, is what type of bike — which is dictated by the terrain you have available and would like to ride. There are subcategories, but a useful rough split would be: a cross-country (XC) bike for good pedaling performance both uphill and on the flat, but not so capable going downhill and in the rough stuff; an enduro if you want to ride rougher downhill terrain (roots, rock gardens, etc.), but still have a bike that pedals well enough up technical climbs and fire roads; and a downhill bike if you want to ride the roughest, most technical terrain, with the steepest descents, and you have access to lift-served mountains (not recommended to start with ;).
FWIW, I’ve had a such a great time riding the Canyon Torque this summer. It’s a long-travel enduro, really capable on the descents, and perfectly fine to get back to the top.
As for motivation, I would echo the sibling comment. Time in nature, good exercise, a ton of techniques to learn and master, a really sociable scene, such a rush riding tough and steep stuff. You won’t regret it.
I'd actually recommend buying from a local shop for your first bike, if there's a good one close by. The warranty service, plus discounts on gear and free tune ups, will be worth about as much as the discount on a DTC bike.
Giant is a good value brand. They make a lot of components in house, so you get cheaper, "good enough" handlebars etc.
In that brand, I'd recommend a used trance 29 or a new trance x 29, depending on your budget. I'm happy to talk about bikes for days!
Trek, Norco, Rocky mountain are also good value. Just about any modern trail bike is excellent.
Also consider a gravel bike.
Currently, it's my favourite type of bike. You do not have to stick to paved roads, which are infested with unattentive/aggressive drivers.
A gravel bike with wide enough tires is a good compromise between off-road capabilities and speed.
If I had decent mtb trails near my doorstep maybe I would use the mtb more often.
My road bike does not see much use anymore.
