Originally Posted by cat4ever
Think about it. Is an hour of training going to help with a (potentially) 4 hour ride?
In a word, probably. An intense 1-hour spin class will improve the aerobic system, and aerobic fitness will determine the pace the OP will be able to maintain over 100k. The better the fitness, the less time in the saddle, and the less chance of suffering.
Originally Posted by cat4ever
Keep spinning, as it helps your aerobic system, but it's not going to help your endurance.
That depends on what's meant by "endurance". If it's the ability to produce a given power output over several hours, then those 1-hour training sessions will be beneficial. But if "endurance" is the ability to be on a bike for several hours without getting a stiff neck and sore lower back, then no, a spin class won't help there. Both types of endurance are important on long rides.