I assume that you understood (and you agree) that as we go back in history, the marathons were wrong measured, inaccurate measured, measured with a old method that did result in shorter to what was today.
This is the main reason why Ron marathon pb is from 69. However he did great marathon results after 72, and i insist, with less training to what he used to do from early career up to 72 olympics.
By "delay" i don´t want to mean simply extend. I want to mean to extend the career but also to extend the career by replicate, by repeat, by approach the high level of performance for a long period.
Someone can extend his career but late part is jut performance decline, but other one can extend his career maintaining the same level of high performances he reached once, or ever get better performances.
Just think about Carlos Lopes, the example i know by memory. He did progress his own 10000m pb every new season from 22years old , start with 29:12 and finish in 84 with 27:17. With 25 years (1972) old he was eliminated from the heats in the Olympics. 4 years later he did win the World Cross Country, and was 2nd in the 10000m in the 76 Olympics an did 27:42 pb in 10000m. In the year of 84, he did win the World Cross country once again, did beat the10000m world record of Henry Rono with 27:17, simply he did win that run, and late season did win the marathon in the Olympics. All this with 36 years old. One year later, in 1984, with 37 years old he did win the World Cross country once again, and the break the marathon world record in 2h07:12. This is what i name career extend by sustained performance progress that i name "delay" (sorry if that is not the right meaning of the word delay).
I keep with my idea be what you say or what i say about Ron Hill training. Ron Hill is one runner i admire, due to his character and run achievements, but i think he did train wrong. He did train the way it was in fashion at that period as marathon training, that is basically to run mileage mostly done by continuous runs but fast, most daily runs faster that the individual marathon pb, and might be some interval quarters as you say 10X400m. Clayton did that too, and some others from that period. I guess they would achieve better results with different training, better training in my opinion.
I did talk personally with Ron Hill in 75 precisely, and he told me at that point after 72 he gave up to run twice a day, because he got no time or interest.
I´m a testimony that during the period he was here in Lisbon-Portugal in 75, he did train just once per day, some 45min - again fast pace as his marathon pb.