@brandon_
OK, that's some good information.
If you can do a full 10 mile hard ride, you will not have a problem from an
aerobic perspective with 60 mile days. Say you guys ride 6 hours total per day (not including time for lunch); that's only 10 mph average to hit 60 miles. Your lungs can do that easily. What you need to work on from now until May is training the rest of your body to do long rides.
Forget about your lungs for a bit. Your backside is going to need to acclimate to being in the saddle for very long periods of time. All the lung capacity in the world won't do any good if you end up in searing pain due to your bike or other factors. You need to find out now if adjustments are needed, otherwise you are going to hate life after the first day.
First, if you don't already have dedicated bicycle shorts with the padding, you need to buy two pairs as soon as possible and get used to riding in them. Six hours per day in the saddle is a long time. The padding will help immensely, and the spandex material will prevent skin damage from chafing (more on that later). Regular shorts will not help (and may hurt if you are wearing abrasive fabrics such as denim).
You need more than one pair of shorts because you will be riding for three or more days. Unless you are planning on doing laundry every night I don't recommend riding consecutive days in the same sweaty shorts.
Smell aside, you don't need bacteria building up in your shorts over time. Wear one pair and let the other pair air out while you ride by lashing them to your rack (this will not work well if you encounter rain).
Second, skin chafing can kill a tour very quickly. Your thighs are not used to constant abrasion for 6 hour days. Even if you have bicycle shorts, I recommend you buy a box of
chamois cream, and use one packet at the start of each day.
Third, you need to figure out if the seat on your current bike will cut it for touring. In most instances you will not have to buy a new seat, assuming you take the measures mentioned above and that your bike's seat was properly fitted for you upon purchase. In extreme cases, that narrow carbon fiber seat on your MTB might be light and nice for trail blasts, but not so much for long tours. If you were not fitted for your seat, ask your local bike shop if they have the special device to measure your sit bones. It's basically a device with a gel pad that helps you determine what width seat is best for you. If you can't get fitted, don't worry about it because you should always do a series of long test rides (more on that later) to see if your current equipment will work.
Fourth, the same goes for your shoes. In particular, if you are using competition-oriented clipless shoes they may not agree with your feet when used during touring. Those carbon fiber S-Works racing shoes are great for transferring power during climbs but you might develop blisters or hot spots if you use them for touring. If you must use clipless pedals, I recommend sneaker-type SPD pedals that are designed for more casual use or mountain biking. Even if you are using toe clips or flats, you need to ensure that your shoes won't give you issues. Again, you should be doing test rides to figure out these issues.
Further, if it rains getting shoes to dry out overnight is difficult while riding and camping. You have a buddy with a van so weight should not be a concern. Pack a spare set of shoes -- if for nothing else, you can muck around the campsite or use them to go into town for dinner instead of clicking around in SPD/Look cleats all the time. If your primary set of shoes gets soaked you can ride with your spare set and try to air out the first pair on the back of your rack while riding. If the weather forecast calls for frequent rain, you might want to make your second pair of shoes a set of Teva-style sandals (not flip-flops) so you can ride or walk without socks. Wet feet can be dried out much more easily than wet socks and shoes ....
Fifth, you might have other muscle/joint issues (unrelated to aerobic capacity) that you need to identify prior to leaving on your first tour. The muscles and joints you will grind away for a slow 60 day slog are much different than a 10 mile MTB blast. During your test rides you should keep an eye out for any discomfort that pops up, and ensure that you stretch well before each day. As an example, you might get some pain in one ankle during long rides that will never show up during a short blast ride. Those nagging pains might arise from ill-fitting equipment (e.g. clipless pedals that are set up incorrectly), stiff muscles or any other host of issues -- the point is to identify them before the tour to ensure that you can fix them.
Regarding the test ride -- in all cases, I suggest doing a few 20 and 30 mile slow rides on weekends to see how you feel, with your current bike and gear. You'll find out quickly if you can double that over three days without running into issues. If you encounter problems, you can fix them before your tour.
On gear, I recommend going with a free-standing tent. As mentioned already there are some campsites on the GAP where it is not possible to find somewhere to lash a hammock. You have a buddy with a van so the weight of a tent, sleeping pad, sleeping bag and pillow should not be a concern. This is your first tour so you should try to make this part as easy as possible. You can do the ultra-light thing another time.
Make sure you bring a headlamp if you don't want to bring a bike-specific light. You will not need a headlamp for The Big Savage Tunnel as there are a few overhead lights. You will need a headlight to get through the dark when riding to/from dinner and also for camping. As an example, if you camp at the Husky Haven Campground, you will be across the river (and at least 1/4 mile ride) from the restaurants in Rockwood and it could be pitch black when you ride back to the campsite. You also might encounter difficulties on the trail (e.g. flat tires, broken spokes) which cause long delays and force you to ride in darkness to reach your next campground. Don't rely on hand-held lights for biking. You probably already have a headlamp for camping so bring it along.
I'm going to assume that you already have a standard bike repair kit and enough skills to solve normal bike issues (mostly flat tires). If you haven't tuned up your bike in a while, it's a good idea to take it to the bike shop before your tour so they can get it set up properly. There are bike shops along the trail but it can be a long walk between towns.
I also recommend over-loading on water so ensure that your bike has at least two bike bottle cages and keep those bottles filled. Unlike the C&O (which has free water pumps available at each hiker/biker campsite along its route), there are relatively few public water fountains on the GAP. You should ensure that you are topping off your bottles whenever possible, mostly in the towns where you are stopping for lunch and dinner. This is especially important if it is very hot and humid during your trip. I have experienced mild heat exhaustion (despite over-hydrating) in bike touring situations and you want to avoid that situation where possible. You should also be taking salt tablets or other electrolytes with your water in hot conditions.
If you let us know what type of bike/tires you will be using, other gear recommendations can be made. I think you'll be fine as long as you're not riding a road bike on skinny tires.
Regarding the Amtrak, it is good that your buddy has things covered, but you should confirm your departure/arrival times. My recollection is that the westbound train to PGH arrives VERY late at night (around midnight). It has been many years since I lived in Pittsburgh but I also recall that the area between the start of the GAP in PGH and the Amtrak station may not be the best of areas. You don't want to arrive on the train at zero dark thirty and have to ride through not-so-nice areas of town. If my memory is correct about the midnight arrival time you do not want to camp the first night -- you should go straight to your hotel and hit the trail the next day. Make sure you get clarity on this situation, otherwise you will be riding in the dark very late at night!
On camping reservations your buddy is probably correct that reservations are not
required at many of the campsites that I mentioned. That said, you guys are going during near peak season so you run the slight risk of the campsites being full. From memory the campsites I mentioned had decent capacity (especially Husky Haven), with less spaces being available at the Festival Park and the Cumberland YMCA. I'll leave that issue of logistics to your buddy.
Best of luck, you should have a great time. I've always enjoyed my trips on the GAP and C&O (and they have NOT all gone well). It's a valuable recreation area that Pennsylvania can offer and you should take full advantage of it!