Merckx never went for the 5k record (that's the pursuit record, okay?).
Strictly for the 10k and the 20k. Even though this is a great achievement as such and he started very fast indeed, we have to bear in mind that he had the advantage of altitude for these two records. Ritter got them in a separate attempt from his Hour record and in Zurich at low altitude.
The advance in training methods and diet since the 1970's is WAY overrated. Coppi brought to cycling pretty much all the modern methods that current riders need. Only technology improved a lot. Interval training was already generalized in the mid-sixties (though it's of no use for the Hour contest, but it's an indication, people tend to romanticize the past too much).
Merckx was 27 when he broke the hour but if you look at the Hour's history, it's pretty much exceptional to do it at that age. Prior to him, all the best broke it at a very young age (Coppi, Anquetil, Baldini, Rivière) or past their time (Anquetil's second attempt) because at that age they did not have such a heavy calendar on the road and they could prepare for it. Ritter and Bracke broke it in their prime but they were primarily track riders and they had time to prepare for it.
Bobet made an attempt to break Coppi's record by 1954 (at age 29) and he failed because he could not prepare for it. He had just been World Champion and won the GP des Nations and as J-P Ollivier said "you don't improvise an Hour record."
Merckx broke the Hour on October 25 1972. On October 7 he won the Tour of Lombardy. On October 8 he won Across Lausanne, the 2 stages and a criterium in Sallanche and on October 11 he won the Baracchi Trophy. In the meantime he had carried out some testings in labs in Milan and Liège to test whether he could adapt to high altitude (because he knew that Bracke failed in his attempt to grab the title back from Ritter, because he couldn't adapt) but he was all okay.
When Merckx arrived in Mexico it rained cats and dogs and he was feeling a little depressed. "Traveling 7,000km and having the same weather as in Belgium." He had to wait for 5 days before he could start and the track was still damp. Albani said that his body was already weakening by then. A record at high altitude should be either be attempted immediately or after a long stay at high altitude (like Ritter did). This fact and his lightning fast start showed that he could have set a far higher mark.