You need to be good at long-distance ride, camping and cooking (if they are your choices), and bike repair (for self-supported rides).
Long-distance ride: A friend said that there is no way you can train yourself for a coast-to-coast ride with long rides day after day for more than a month; the first two weeks of your trip is your training. There is a certain truth in this, especially for young people. But I’ve seen someone who took this advice seriously, didn’t train, and suffered dearly during a long-distance bike trip. Training would make your trip less suffering and more enjoyable. It is indeed the day-after-day ride for many days that is hard for anyone who is not used to it. It is not just your legs, it’s also your butt (saddle sore – see this article). On your training ride, you carry all the weight in your bags that you expect to carry on the trip (put in books if you are still searching for your camping gears), cover the distance that you expect to cover each day, and climb the hills with the grades (vertical vs horizontal distance) your expect to climb on the trip. Most of us can’t train continuously for many days, but you should train at least several days in a roll to have a taste of riding day after day. Most major roads in the US climbs less than 7%. That should be your training target (at least two miles continuously). All you need to do is to shift to your lowest gear and go slowly. There are peddling techniques for climbing (see this article). When a hill is too steep to ride, there is no shame to hike it (walking and pushing your bike).
Camping/cooking: If you know backpacking, you don’t need to train for this. Otherwise, you need to practice setting up and packing your tent till you can do it without thinking and even in the dark without losing any part. You need to decide the level of cooking you want. It can be as simple as boiling water to make soups or noodles (my choice) to as complicated as gourmet meals (need to wash your cookware!).
Bike repair: Every biker should know how to fix a flat. Alas, there are so many who ride without this skill. Tubeless tires (without inner tubes, sealant inside the tires fill small punctures) can save a lot of trouble. When you do have a flat on a tubeless tire, you can always treat it as a regular tire by inserting an inner tube. You would benefit from frequent cleaning and lubricating the chain (in a correct way by dropping lube only on the chain joints). You may find your shifting needs to be tuned on the road. These skills are not hard to learn, and many local cycling clubs offer their training classes. Beyond them, skills for bike repair on the road are boundless and it’s hard to say what are must to have. Experienced bikepackers can tell you that their bikes broke on any possible parts. Someone had to get new bikes on the road. A good advice is to bring common spare parts (chain links, derailleur hanger, spokes, brake/shifting cables, brake pads, etc.) even you don’t know how to use them. Someone you meet on the road may help if you have the parts. Having a reliable bike well serviced by a bike shop before your trip would go a long way.
Chidong Zhang's WA2MA Bike Trip 