Actually a very good question, and there are various ways to go about answering it.
If by endurance work you refer to what amount of mileage needs to be done to transform a 2:05 runner to a 15 flat 5k runner, well the answer is that volume is undefined. No specific *amount* of training will ensure the result.
With training, the improved ability to maintain speed can be quantified in various ways.
Physiologists have been known to quantify endurance gains just as they might quantify gains in mitochondrial content: 2 fold, 3 fold etc, or V02peak, or VT example 20% gain in 3 weeks. The problem is you have to select a velocity to make that comparison pre/post training and the velocity you choose will give you different answers. For example if I advise a runner from a 20' 5k to running 42k at that same speed, I could say the endurance gain has been greater than 8x at 4'/k pace. But that same runner might have just a 2x increase in endurance if we consider 2:30/km (30" 200m pre to 400m 60" post). And absolutely no improvement in endurance at 2'/km following training because before and after training, 2'/km pace is impossible!
The second way uses the velocity-duration curve, which can be transformed mathematically by applying techniques from enzyme kinetics in chemistry, to give a series of linear equations, y = mx +b. The change in the slope of the that line with training is a very valid measure of endurance gains. In truth the entire velocity duration curve consists of three separate lines, but the most vital aspect would cover peak running speed to roughly 2000m running speed for a 2:05 runner.
The third way to assess endurance gains with training is to compare the ability to maintain a relative speed following training, comparing speed for one distance to another. Figure if endurance is getting better you'll run relatively faster for the longer distance. When looking at performance improvements this way, you discover the improvement in a race like 5k is typically proportional to gains in speed for shorter distances. If a HS runner runs 2:05 for 800m and 16' flat for 5000m, then after 8 years training runs 1:52.5 and 14:24' 5000m, in one respect the endurance is not really improved at all. 5000m time is still exactly proportional to the 800m time. 5k in both cases, is ~7.7x 800m speed.
At the top levels of the sport we find that the best 5k runners -- in relative terms at least-- often have so-so to average endurance and rather spectacular speed for shorter distances. So when considering relative speeds, a slug can have better endurance than a cheetah.