http://www.stuff.co.nz/the-press/sport/6365084/Peerless-Snells-Christchurch-records-still-stand"From 1961 to 1964, (Peter) Snell ran '100 miles a week with me' for two eight-week blocks each year. In 1964 when he won the 800m and 1500m double at the Tokyo Olympics, Snell ran 160km (100 miles) a week for 10 weeks in a row, Magee said.
Seven weeks before he broke three world records in eight days – the mile record at Cook's Gardens in Whanganui preceded his Christchurch feats – SNELL RAN A MARATHON IN 2 HR 41 MIN."
As far as I can tell, Snell does not have a faster recorded marathon time than this 2:41.
So Lydiard's star pupil did not break 2:30 while putting in 100 mile weeks just two months before breaking multiple world records in the half mile and mile. Obviously, Snell may not have ran all out for that marathon but I doubt he ran a recorded time without putting forth a decent effort.
The fact that people with ordinary leg speed can break 2:30 is no more surprising than the fact that people with ordinary endurance can run sub 11s 100 m or high jump 6'-0". Yes, there are ordinary milers who can run sub 2:30 marathoners but that is because they are above average to extra-ordinary marathoners.