They can be windy smooth trails through the forest, or rocky technical trails to test your bike handling, or gnarly steep downhill trails, or jump trails, the list goes on. You have full control over what you get yourself into, and I can just barely scratch the surface of the variety of riding available across all the disciplines and locations. The basics are the same, you and your bike on some trails.

You can develop your bike skills and challenge yourself on fast or technical trails, or you can just take it easy on flowy easy trails or cross country trails. You can dial up the adrenaline as much or as little as you want, and the terrain can be vary wildly from trail to trail so it's always new and interesting. You'll end up riding parts of your locale you'd never imagine and if you go travelling with it, there are trails in rainforests, deserts, mountain ranges, rolling hills, all over.

For the fitness side, you build strength and dexterity across your whole body as you ride more, and the ride up the hill will develop your cardio and often be interesting trails of their own.

Check out the Trailforks site/app to find trails near you, go rent a bike for ~$50 for the day from any MTB store. It's very easy to try without spending much.

The only equipment you need is a helmet, any bike helmet will do for your first rides. You'll figure out the rest of the gear as you go. Modern bikes are so easy to ride and have made it a crazy fun.

I also suggest XC / blue / "intermediate" / rolling trails. They're less exposed and dangerous than steep descents, and better cardio. The MTB industry and press is increasingly downhill-oriented, but you can ignore that noise. Before you buy a bike, try renting a few, and riding them on your own local trails. It'll give you a better idea of what sort of bike and riding you want. If you don't have much singletrack nearby but you have dirt roads, consider a gravel or "all-terrain" road bike instead (like a Kona Dew, Kona Rove, or Hudski Doggler). It's not mountain biking, but still a lot of fun, and you cover more ground. (They're also simpler and cheaper than modern mountain bikes.)

Bike fit is important, take your time to get it right. Don't buy the wrong frame size! (I've made that mistake at least twice.) Find a shop (or experienced friend) to help you dial in the fit, and expect to swap some parts. This (and physical conditioning) makes the difference between riding kind of hurting, and being comfy on the bike for 2+ hours.

Obviously you can get hurt. MTB injuries tend to be less severe than road cycling, because you aren't riding in car traffic. But several years ago I tore out my triceps, going over the front of the bike in a rock garden and landing on my elbow. That was a surgery and slow recovery. I'm fine now, and even with that injury, cycling has been a benefit to my health and enjoyment of life.

It can be a gear-intensive hobby. I have a mostly-bikes room. It can also be an expensive hobby (though doesn't need to be). Still much cheaper than motorcycles, sailing, and so forth.

Sorry, one last thing. Be courteous to hikers. Yield to them by default, call out if approaching them from behind. Making hikers feel safe and respected is more important than carrying your speed. When riders forget this, angry hikers show up to meetings and agitate for closing trails to bikes.