Congrats Rocky. I'm glad we could provide some input and I must say, you're a rare breed of newcomer that pauses and listens before pulling the trigger. I'm sure it was painful both financially and will be physically once you get started rolling on dirt, but it will be worth it.
A few things that haven't been said yet:
-This sport hurts physically, I think most of us have made that clear either directly or indirectly. That's the bad news. The good news is that it's surprisingly low impact considering the adrenaline it delivers and the fitness you'll gain. Since it doesn't hurt like some traditional workouts (lifting weights) it becomes more of a mental battle of how far you're willing to push yourself and how far down the pain cave you're willing to send yourself. They key to remember is that it hurts for everyone. When you're struggling to push your bike up a hill and some one motors right by you, know that they were absolutely, where you are today at one point in their riding career. The pain never goes away, but your threshold and tolerence and enjoyment of the pain does. The thing that is consistent is that the pain/suffering is one of the things you come to love, and those that have been doing it for a long while, feel the same way you do on nearly every ride, they just may have to spend another 60-120-180-240 minutes on the bike to get there. In short, we're all in the pain cave together.
-Pay attention to the little gains. This sport is going to be defeating and brutal at first. Some people fizzle out because it takes too much effort. The thing that's important is to look at your progress and marvel in the small gains. 2 months in, sure you may not be able to do a 4 hour epic ride like some... but take a step back and notice the little rock feature you can handle that you couldn't when you started or the climb that once had you huffing/puffing and walking that you can now motor up. Don't forget to look at the big picture and the small picture. Baby steps and before you know it, you'll be passing hours by like they are minutes on the trail.
-Take a lesson and/or ride with experienced people and get all the pointers you can. This is complicated sport. If you've ever done downhill skiing, water skiing, motor cycle riding, sporting clays, hunting etc, this is like that. Get comfortable on your bike and get 6 months of rides under your belt and get some pointers. It will unlock huge new things for you.
-This is a mental game. While the sport may seem physical to you right now, and I'm certain that is the overwhelming struggle at the moment, it's 100% a mental sport. Your body is already more capable then you know it is. It will take you just as long to find the limits of your current state as it will to hone and built the future more fit and capable you. Self confidence is huge and it's a huge wall for people to smash through. The skills needed to negotiate our local terrain are difficult, comprehensive and take a while to master. If you're getting shaken or being intimidated by a specific feature, decent or climb... slow down and break down what is affecting you. Is it how loose the ground is? Is it a rocky part with a turn right after? The important thing is to break it down and think through what it is that digging at your psyche and tackle that thing or issue one part at a time. For example, loose section coming into a corner? Think through where your body wants to be in each segment of trail and envision how you think it should go in your head and try to execute. You will likely fall short the first few times (or years) but with time, you'll be able to put some of the core movements and reactions on cruise control allowing you to focus on higher level tasks. It just takes time in grade.
-Stick with it. If you're doing this to build fitness... you've come to the right place. I know tons AND I MEAN FREEKIN TONS (MORE THEN I CAN REMEMBER OR COUNT) of people that have changed their lives and achieved their fitness goals through bikes, specifically mountain biking. I know people that have lost 100lbs in a year making a small change to their diet and jumping on the bike 3x a week for 1 hour each time. I know people (like myself) that at an already slim athletic frame, can make significant changes in body composition in a matter of 4-6 months just cutting beer and riding 3x a week. This sport will peel the weight off you and make you fitter then you've ever been in your life. I don't care who you are, this fact is universal.
-Be goal driven not outcome driven. A big part of sticking with this is being goal driven and not consumed by the outcome. Like many things in life, there are too many variables out of your control for you to have a real say in the outcome. Even the most experienced among us set out to do an epic ride, only to be sidelined with a mechanical that is ride ending that's not in our control. For that reason, chose goals that give yourself the best chance to achieve your outcome, and don't focus so much on the outcome. The best summary of this is what you see mtb riders preaching all the time... "The worst day on the bike, is still better than (chose your cliche thing here)" That's because it's true. In this sport, the outcome is just another notch in your belt... the most fulfilling part is the 10 other notches that were the goals your achieved on the way to that outcome. Love every part of it, that's what this is about.t