When I was trying to integrate a definition of “flow” that covered my many experiences, it wasn’t biking up hills per se, it was those moments where bad shit almost happened and I saw a way through and just did it. Riding through gravel, loose dogs, when the “fast guy” tried to drop the pack and found me on his wheel.
I think I agree with you that these experiences are pivotal, because they are moments when you just are without thinking. That constant monologue some of us carrying our heads is gone, and the universe didn’t end.
Taking that information and finding the spot halfway between the two is the real lesson, IME.
This is what mountain biking does for me. It's not hard, and it's pretty safe, but maintaining decent speed while navigating trails... it's a flow state.
Heck, a week ago I did a 100 mile MTB race, with the goal of finishing and being happy. While bits and pieces are there, some chatting with folks and things I saw along the trail, but I honestly do not remember most of the ~9 hours I was riding. It was just a thing I was doing, disconnected from most of the rest of the world, and content doing it.
(Note, I'm talking about XC/cross-country mountain biking, and to some extent trail biking. NOT the big downhill/jump/whatever stuff that comes up most often in media when sees mountain biking. It's still quite exciting, but far safer because it's really just riding bikes on hiking-ish trails.)
MTB has pretty much all the good stuff in one. You're outside, in nature, breathing fresh air, it's cardio and mindfulness on the climbs and flow and focus on the descent, add adrenaline to taste.
The community is pretty rad too, it has the atmosphere of surfing and hiking combined.
A typical ride starts at the trailhead where you meet the people you're riding with, or not (I ride solo a lot). Then you ride up a trail that takes you to the fun stuff, the decent trails. All trails are graded with a difficulty, so you pick one that suits you and start the decent.
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.
The ancestor comments are spot on. I just wish it didn't require driving. The good stuff is 10+ miles from where I live. Not that far, but it's still burning gas just to ride a bike.
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.
Great advice allround, especially on gravel and XC. They still have all the same pros as MTB, different types of riding but very much adjacent.
I am lucky enough to live within riding distance of a trail centre, they aren't the best trails but having any trails nearby in a suburban area is amazing. It's a 10 minute pedal and I get out once or twice a week to those trails.
Something else, compared to road riding, is that most mountain biking is very safe. Typically you are riding fairly slow on dirt surfaces that you can stop on. Basic falls (falling over) are common, but increasingly rare as you get more experienced. There generally aren't other people posing you risk as there is in group or solo road riding, it's just you and nature.
It's the "flow", as with many other sports requiring very high levels of hand-eye coordination, whether climbing, martial arts, olympic weight lifting, fencing etc. You don't have time to see, mentally process, then do; instead you just think/feel and the body does, and somehow that is much faster than is possible by the physical measurement of electrical signals from your eye to your body.
I think I agree with you that these experiences are pivotal, because they are moments when you just are without thinking. That constant monologue some of us carrying our heads is gone, and the universe didn’t end.
Taking that information and finding the spot halfway between the two is the real lesson, IME.