Look for carbon. You need to be comfortable for long periods when distance cycling. Carbon fiber bikes absorb road vibration that causes rider fatigue. If you can't afford all carbon, buy a bike with as many carbon fiber parts on it as you can afford.
Look for handlebars that have wide, flat tops on them, with ergo designs where you can keep changing hand positions throughout the day. Changing hand positions also changes your spine position, alleviating distance fatigue.
Look for a seat that has a channel down the middle commonly referred to as the "love channel." The love channel is an ergonomic design that allows for the comfort of the cyclist's private parts.
Do not buy a bike with a wide, flat seat. Wide, flat bicycle seats damage and irritate the soft tissue on the inside of your thighs. Also, avoid handlebars that are elevated higher than your seat. High rising handlebars sit the rider in a less than aerodynamic position on the bike, resulting in slow, inefficient distance traveling.
Where to Buy
It's best to buy your bike at a bike shop. They can back up your bike with the proper maintenance. The best place nationwide to buy a distance bike is REI. They have one of the largest selections and best maintenance shops. For best prices, you can buy your distance bike online through Bike Nashbar or Performance Cycling.
Some long distance touring riders are comfortable on a steel touring bike that costs about $400. Most average cyclists are OK with entry level road bikes that are part aluminum and part carbon fiber that run about $800 to $1,000. For the hard-core distance cyclist, plush all-carbon bikes will set you back $2,000 to $3,000.
There are three types of bikes that are routinely used for distance cycling. The touring bike is the heaviest, and sits the rider in an upright position. Touring bikes are slower, but can carry extra weight. Recumbent bikes have a configuration that allows the rider to sit in a seat that supports the rider's back. Recumbent bikes are good for cyclists with bad backs. Recumbent bikes are slightly slower than upright bikes. Traditional road bikes are the fastest, but the rider sits bent over forward. This position is the most efficient.
Every long distance rider should be equipped with an underseat pack containing at least one spare tube, one tube patch kit, tire levers to remove the tire and a universal tool that fits most of the fittings on the bike in case your bike breaks down on the highway. Another vital item is a compact air pump. These small pumps attach to your water bottle cage. Use them to pump up your tire if it goes flat. Don't ever leave home without a good insulated water bottle. An underseat pack costs about $15. Tubes and tools cost about $20. A compact air pump is about $20. And an insulated water bottle is about $10.
A small amount of saddle-soreness will always occur when beginning a cycling training program; this will go away. Even experienced cyclists have some saddle soreness the first few minutes of cycling. Stick with it and soon you will be completing your first century ride of 100 miles, a benchmark for cyclists. You should finish your first century pleasantly exhausted, but more satisfied with yourself than you have ever been in your life.