You didn't say what your average miles you ride on your mountain bike, and whether or not the terrain you ride the MTB is mostly off road vs road, so exercise beforehand may not even be necessary if you riding pretty good numbers on the MTB. Look at this way, roughly 20 to 25 miles on a MTB off road is equivalent to about 50 to 55 miles on a road, that should give you a clue as to how far along you are. If you feel uncertain about your level of riding with a MTB and feel you need to improve by training then check out this site:
http://www.kintera.org/htmlcontent.asp?cid=54755 You have to follow this plan pretty much to the letter, but if you noticed this a 100 mile ride plan however since you'll be doing 68 miles you only really need to work up to the first 5 weeks, on the Saturday of the 5th week you simply do your 68 mile ride instead of a 53, or if you want additional time then do the 68 mile ride on Saturday of the 7th week. The word Brisk in that chart means you should do intervals IF you want to do the 68 mile trip faster. Intervals work like this basically, there are other methods but this is one of them; you warm up for about 5 minutes at an easy pace, then for 30 seconds you pedal very hard at a high cadence in a hard gear, then you back it off to an easy pace for 1 minute then repeat; this cycle should be performed 5 to 10 times, 5 times the first week, 6 times the second week, etc. If you have a heart rate monitor what your looking at with the hard outputs is a zone 4 workout which is about 60 to 70 percent of you max, the easy pace gets your heart back down to zone 3 or 2 (whichever occurs last at that 1 minute easy pace) before you start to crank it back up again.
I wouldn't necessarily take all Gatorade, for some that can get rough on the stomach, I would take about 1/2 water and 1/2 Gatorade, I would also dilute the Gatorade about 50% for better absorption without upsetting the stomach; then what you do is take a gulp of Gatorade followed by a gulp of clear water this will wash some of the Gatorade from the mouth so you mouth doesn't feel stick and gooey as the ride progresses. If during the training I mentioned above you find out you're having trouble stomaching the Gatorade, some people do some don't, then try a different drink mix like Cytomax, but first try Gatorade because it is cheaper and works the same, even with Cytomax you should still dilute it 50% strength from the recommended dosage. You need to be drinking a bottle about every 45 minutes to an hour depending on how hot it is, which means you'll need 3 bottles, the third bottle should be a mix. These bottles if they're 27 ounce bottles, on a mild day can last 3 1/2 hours, meaning the last 1/2 hour you won't have a drink but that's ok because you'll be home and can drink then, but on hot days you'll have to stop at a convenience store and refill with some sort of hydration drink (you can even gulp down a small can of V8 juice at the store and take the hydration drink along for the ride). If you're using smaller bottles you'll for sure will have to stop even on mild days to get more drink, or if you don't want to mount a 3rd bottle you'll have to stop as well.
On the subject of drinking, the colder you can maintain the fluid the better it is for you because the cooler drink temperature brings down your core body heat which is good, but you can't keep a regular bottle drink cool for more than an hour then they all will be hot, so what do you do you scream? Get 2 or 3 27 ounce Polar Bottles, these are the best plastic insulated bottles on the market. With 3 this works great, you put a Gatorade bottle in the freezer for 48 hours before the ride, allow about an 1/2 inch from the top for expansion, along with that bottle you put your empty Polar bottle(s) in the freezer. Then you put one bottles worth of Gatorade and one bottles worth of water in the frig for the same 48 hour duration. Then just before you leave you take the stuff out of the freezer, you fill one bottle to the brim with ice then pour the chilled Gatorade into it, fill the other bottle with 2/3rds ice and fill with water, place on bike and go. Obviously you'll be drinking from the bottle with the ice cubes first and the frozen bottle last; don't do this on cool cloudy days or the frozen bottle may not unfreeze enough to drink it!
Also you should drink 16 ounces of full strength Gatorade about 2 hours before the ride, then 1 16 ounce of water about an hour before the ride; then pee before you leave.
The other thing to take that is popular for energy are the various gels on the market, but that stuff is expensive and doesn't do anything different than just good ol strong coffee, problem is how do you carry it? well over the years GU has made flask to pour in their gels, some were mounted with a small plastic cage that mounted to the frame, later they went to a cloth holder that held with velcro straps, now they just have a soft sided flask you can put in your jersey pocket, they cost about $12 on Amazon but can be found at most LBS's. Simply make some very strong black coffee, don't put sugar or cream in it or you could upset your stomach, and take a small sip of the coffee about an hour into the ride and then every half hour after that, if you plan it right you should finish the flask about a 1/2 hour before the ride ends.
Food is a personal thing, a lot of people are fine with Cliff bars, but they don't work for me, I burn through my food rather quickly so I have to eat a Met Rx Big 100 Colossal bar from Walmart. I cut the bar into 1/2 and put them in a plastic zip lock bag, depending on how far I'm going of course and how hard my effort will be will depend on how many bars I'll take, but on a moderate ride of 60 miles I'll take one bar, then about a third of the way into the ride I eat a section along with the sip of coffee, and a gulp of Cytomax and a gulp of water, then about 2/3rds into the ride I eat the other half of the bar along with the other stuff. Some people will divide their bars into 6ths and eat a 6th every 1/2 hour, you have to see what works for you.
All of this stuff I wrote is just an opinion, there are many other ways to do it, most will not be wrong.
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