Is the aerobic system necessary to help recover during longer anaerobic races?
The aerobic system, in particular the Type I muscle fibers, are responsible for processing the majority of the waste produced by the anaerobic fibers, and to replenish their fuel. This means that there is no way to train the anaerobic muscle fibers to recover better---"recovery" is not in their job description. In essence, your ability to recover from aerobic OR anaerobic workouts is entirely dependent on your aerobic system.
For this reason, it is even more important to have a very powerful aerobic system, especially for really long races, regardless of the race topology. (When the race topology has a lot of hills or difficult terrain, etc, it's important to ALSO have powerful anaerobic muscles).