Is endurance the ability to ride for 5 hours at 19 mph? Is endurance the ability to follow the 43rd attack in the bike race? Is endurance the ability to ride 8 match sprints in a day? What do you think?
I would suggest that endurance is all three.
Riding all day at a moderate tempo I define as base endurance. Without your base endurance, any hard work you do will over-tax you very easily. Base endurance is the type of training everyone loves.. In fact, everyone loves it too much. At one point in time it became an entire training regime. It used to be know as LSD. (Long Slow Distance) This is the work that lays the base in December, January and February. Miles, miles and more miles. This time of the year those long miles at low intensity with friends are the basis for the rest of the year. The local hammerfest is not appropriate here. If your training partners get a warm day and decide it is big chainring time...wave bye-bye...let them go up the road while you continue your ride.
Having the endurance to do repeated efforts in the criterium, circuit race or points race is developed by doing interval training. Intervals are simply putting the body under a high load for a period of time, unloading for less-than-complete recovery, and repeating until performance falls off more than 10%. That is an interval set. Intervals can be done on the track, road, trainer or rollers. For most riders, interval training should not commence before the last week or two in February. Please keep in mind that intervals are brutally hard training if done at the proper intensity and a ridiculous waste of time if done at a lesser intensity. Never do more than 3 interval sessions a week, and never do more than 3 weeks in a row without a recovery week. Intervals can be stacked up in a more intensive manner, but should only be done so under the guidance of a trusted coach. Remember to be very careful to recover fully every 2-3 days. It is very easy to dig yourself into a very deep hole by doing interval work.
The third type of endurance is the one almost no one but track sprinters ever work on. It is the ability to perform at very high levels multiple times in a period of 2-4 hours. Of course it’s obvious why sprinters need that kind of ability, but why the local criterium rider? Wouldn’t it be nice to win two primes and the field sprint, rather than have to decide which one to go after? You can if you will do workouts that emphasize power and speed efforts of :30 to 1:30 with full recovery between efforts. Most people think full recovery means they are not really training. Full recovery means you are training the body to fully recover quicker. Full recovery means that you are doing efforts at 100%, not the 95-98% you use for intervals. Believe it or not, that 3-5% is the difference between getting faster or not and being able to repeat the fast efforts throughout a race or race meet.
So, three types of endurance. You need all three, You need to train all three. You will be a better bike racer for it.