Lydiard's system as I recall, had runners do 100 miles a week on 1 run a day
The endurance phase would last about 9 months and would consist of.
Monday-10 miles flat
Tuesday-15 miles some hills
Wednesday-12 miles flat
Thursday- 18 miles very hilly
Friday- 10 miles flat
Saturday-15 miles flat
Sunday- 22 miles very hilly
as you get in better condition and get accustomed to this, you will get faster on all runs and the flat runs would get to 5:15-5:30 per mile and the longer hilly runs would be about 6 minutes a mile, including the 22 mile hilly run.
6:00 minute pace comes out to 2:37 for a marathon, so if your following the above schedule, you would just be running a little faster and a little longer than your weekly Sunday long run AND to run 2:30 that would be considerably slower than the pace for your flat runs.
So IF (how big the IF is depends on the person) you can work up to Lydiard's schedule 2:30 should not be a problem.
As to Snell only running 2:41 he ONLY followed the above schedule for 8 weeks (and I believe only 1 time) had he followed this schedule 9 months a year for 5-10 or more years consecutively then Snell certainly could have broken 2:30 EASY and by a lot.
Also Lydiard Included the 22 mile run in ALL phases, so a 22 mile hilly run at 2:37 marathon pace 52 times a year.
Lydiard also recommended a 2nd run a day of 30-60 minutes at a very easy pace to speed the recovery.
Now I don't know if ANY healthy man can work up to this schedule, but if they could 2:30 would be EASY, any person who was working with Lydiard had a above average work ethic, if they were willing to commit to the above workload.
What Lydiard refers to as any healthy man may refer to who he has coached, as that is all he sees, Just like a Division 1 coach with rare exceptions the least talented kid working out with their team was at WORST a 10 minute 2 miler in high school and Most will have run between 8:50-9:20 for 2 miles in high school, when you start with that level of ability and do the above schedule, it would be shocking if they can not run under 2:30 for a marathon even if they are big for a runner (Being smaller is an advantage in the marathon, being bigger is a disadvantage).
What Lydiard calls any healthy man might refer to who he has coached, and not in general, Example I would say any distance runner with a scholarship to The University of Colorado, Wisconsin, Oregon or any college can do it easy,
with the above schedule, but being they have scholarships they are not average, but for lets say Mark Wetmore a kid who runs 9:00 for 2 miles is nothing unusual, a prospect who needs to be developed to be able to compete, A high school coach who has a 9:00 minute 2 miler may think of the same kid as a superstore.
So Lydiard saying Any from his experiences, is not any to the general population.
Sorry if it was too long.
Happy New Year to all.