Cycling 100 miles (a century) is an achievable feat for most people assuming adequate preparation and training are performed.
Obviously the main thing you need to do is to start building up the miles you clock on the bike as early as possible in your training regime. You should aim to be able to complete a 75 mile training ride two weeks before your century attempt. Assuming you achieve this then the extra 25 miles on the day should not be a problem.
Based on this 75 mile plan you will need to slowly build up your rides to this length in order to avoid injury. Aim to go for at least 2 short rides during the week and then one long ride at the weekend. The long ride (and your weekly mileage total) should not increase in distance more than 15% each week. Ramping up your training at a faster rate will significantly increase your chances of picking up an injury.
Another essential way to prepare for your 100 mile ride is to treat your training as a practise ground for the kit, clothing and nutrition you will use on the big day. Try to simulate race circumstances on every training ride by wearing exactly the clothing and carrying exactly the kit you will start with on century day. By doing this you will reduce the risk of any nasty surprises ruining you enjoyment of the 100 mile ride.
Nutrition is key on such a long ride. Use your weekly long rides to experiment with what foods work best for you. Foods such as potatoes, bagels, fig rolls and the famous peanut butter sandwich are all popular amongst endurance cyclists. Find out what works for you and what does in training not on the century attempt itself.
