The Aerobic Capacity tests are what most runners and cyclists would like. The first video with Neville is more instructive and should probably be seen first. The second with Mark shows the same test in a different trial.
This test can tell you 4 main things:
a) what your VO2max or maximum capacity to take in and use oxygen. This basically can tell you "what kind of shape are you in" and compare you to the rest of the population.
b) What your anaerobic thereshold is in terms of an absolute value and as a percentage of your VO2max. This is important to know because some people may be able to get large gains in racing performance with small amounts of higher intensity training. There is a balance between high and low intensity training that this the key.
c) When you are at your maximum fat burning and how to shift that point with training. This is important if you want to lose weight or spare your carbohydrates for the long hard runs.
d) What speed or workload you will be able to maintain for amount of time. This is critical for effective pacing and not "hitting walls" or "bonking" before the end of the race and to track your performance increases or decreases.
Video with Mark A. being tested by Dr. Mark Kelly