What changed, speaking specifically about Marathon, is the "coefficient of resistance".
Look at the PB of 10000m and Marathon of some of the best specialists in the past (probably I do soe mistake, because I write what I remember, here in China I don't have my statistical books....) :
Shorter 27'45"9 (2'46"6/km) 2:10:30 (3'08"6)
ratio : 88%
Zatopek 28'54" (2'53"4) 2:23:03 (3'23"4)
ratio : 85%
Rodgers 28'04" (2'48") 2:09:27 (3'04")
ratio : 91%
Seko 27'42" (2'45"7) 2:08:27 (3'02"6)
ratio : 91%
Lismont 27'56" (2'47"6) 2:11:13 (3'06"5)
ratio : 89,8%
G. Mutai 27'e (2'42") 2:03:02 (2'55")
ratio : 92,5%
W. Kipsang 27'30" (2'45") 2:03:23 (2'55"5)
ratio : 94%
Makau 27'30"e (2'45") 2:03:38 (2'56")
ratio : 93,75%
E. Mutai 27'50" (2'47") 2:03:52 (2'56"5)
ratio : 94,6%
Kebede 27'50" (2'47") 2:04:38 (2'57")
ratio : 94,3%
This clearly shows there was something new in training, and this is the VOLUME OF INTENSITY.
Of course, no new ingredients, but different way to use.
About new ingredients, somebody of the Lydiardists can explain me which new ingredients Arthur put in his training ?
Long run already existed from the time of Alfred Shrubb.
Interval training already existed from Gerschler and Reindell.
Fartlek already existed from the time of Paavo Nurmi, and after Gunder Haegg.
Running uphill already existed from the time of Jean Bouin.
But you speak of him as an "innovator" (and in my opinion you have reason). Why ? Somebody can explain me where his innovation was, if not putting together in different way the ingredients already known?
For example, replacing a normal long run at fast pace with 32 km run with fractions of 7 + 6 + 5 4 + 3 + 2 km, starting from Marathon pace (7 km) for increasing the pace in every fraction of 1"/2", alternated with 1 km NOT JOGGING, but run at 85% of Marathon pace (for example, for an athlete running full Marathon at 3' per km, around 3'25"), is not some "innovation" ? Using 20-25 km running uphill, with a difference in altitude of 2000m from start to finish, is not some "innovation" ? Using 1 hr 30'of fartlek with 10 times 2' fast recovery 1' at 85% of speed + 20 times 1' fast recovery 1' at 85% of speed + 20 times 30" very fast recovering 30" very easy is not some "innovation" ? Using special blocks bringing the athlete to run 50-60 km in the same day, including 45-50 km in the area of specific speed for Marathon, is not some "innovation" ? Using long period of recovery, with higher modulation between intensity and recovery in order to allow the supercompensation to work, is not some "innovation" ?
And now all these element are part of the "normal" training of the best African runners, that are very much more advanced under methodological point of view than European or American, when we speak about Marathon.
How I already said, nothing really new in short distances training, but many things new about long distances.
But, of course, running is Always running, nothing new in putting one foot ahead the other foot.