Here’s another way to look at the situation; now this is based on fiction so the actual numbers may not completely accurately reflect the real life situation. Number are completely “for the argument sake.”
We have runner A and runner B. They both have 55-seconds 400m speed and would like to run 800 in 2 minutes. And both of them start getting lactic acid when you go 10 seconds beyond their best 400m time (this is kind of bogus…); in other words, they will still be “aerobic” while running the speed 10 seconds slower than their best 400m time (you wish…). And both of them can sprint 100m flat out in 12 seconds. Runner A decided to work on his endurance; while runner B decided to hone his speed (now this ain’t gonna happen because even doing interval training all the time WILL improve your aerobic capacity somewhat…). By the race time, runner A developed his aerobic capacity up so now he won’t start getting lactic acid until he runs faster than 62 seconds (7 seconds slower than his best 400m effort). Runner B improved his 400m time down to 53 seconds and, as a result, now his 100m PR is down to 11.5 seconds; but his aerobic capacity remains the same so he will start producing lactic acid at 63 seconds per 400m (53 + 10). Now in the race both of them run the first lap in 60 seconds; 2 seconds faster for runner A’s threshold and 3 seconds faster than runner B’s threshold. It’s only a slight difference in second but we know physiologically that, with the slight increase in speed, lactic acid production shoots up (as Lydiard explained as “it doubles, squares and cubes”). By the time they both come around the last bend, runner B’s legs are full of lactic acid which creates neuromuscular breakdown and it’s very difficult to contract muscles, he struggles to manage only 17 seconds for the last 100m with “wobbly legs” (anybody who runs 800m knows exactly how THAT feels); whereas runner A, while also getting into oxygen debt, the damage is not as much as runner B, also “because he’s marathon-trained, he does not feel as tired,” he can sprint close to flat-out in the last 100m (as Snell displayed number of times), perhaps 13 seconds.
The question is who’s the least anaerobic coming into the last bend. The question is how much aerobic can you stay during the race; how high is your aerobic level (threshold pace) compared to the speed at which you’re running the entire race. This is why, in the international competition with Kenyans and Ethiopians with very high aerobic capacity, they are not as much anaerobic coming into the last lap that they can sprint flat-out while others, even if their basic speed is actually faster than theirs, barely maintain the speed with “wobbly legs” full of lactic acid. Somebody mentioned Bekele’s speed. Yes, he’s very fast even in 400m (sure as hell quite a lot faster than me!) but there are others who can run flat-out 400m against Bekele faster, in fact quite a bit faster. But if they get too far anaerobic by the time they reach the final lap, all the speed in the world is not going to help them out-sprint Bekele. Now THAT is the basic concept of the Lydiard method. And THAT is the reason why you do all those marathon-conditioning to develop your aerobic capacity as high as possible.