This shows that a three-day march could be covered in....how many hours? Well that depends on how many watches there are in a night; the Greeks, like the Jews (the Bible being a largly Hellenistic production), divided the night into three roughly four hour watches whereas the Romans had four watches. Either way, Diodoros must mean 'about the end of the second watch', otherwise it would be possible to cover three days' normal march, say 45 miles, in four hours; 11.25 mph and it is not feasible to quadruple mount! Allowing eight hours the speed necessary drops to 5.625 mph, which is fast but attainable by picked troops interval running and unencumbered save for their weapons. Stretch a day's march to 20 miles, however, and it's 7.5 mph. Average speeds are 3.5 mph walking, jogging varies between 4.5 and 5.5 mph, nobody is going to run for eight hours. As an additional point of reference, a British Marine has to complete a nine mile speed march in 90 minutes, ie 6mph but only for 90 minutes over uneven terrain, and those undertaking this are considerably fitter and better nourished than even 'picked' ancient soldiers.40 1 Antigonus, who had been designated general of Asia for the purpose of finishing the war with Eumenes, collected his troops from their winter quarters.After making preparations for the battle, he set out against Eumenes, who was still in Cappadocia.2 Now one of Eumenes' distinguished commanders named Perdiccas had deserted him and was encamped at a distance of three days' march with the soldiers who had joined him in the mutiny, three thousand infantry and five hundred cavalry. Eumenes, accordingly, sent against him Phoenix of Tenedos with four thousand picked foot-soldiers and a thousand horsemen. 3 After a forced night march Phoenix fell unexpectedly on the deserters at about the second watch of the night, and catching them asleep, took Perdiccas alive and secured control of his troops. 4 Eumenes put to death the leaders who had been most responsible for the desertion, but by distributing the common soldiers among the other troops and treating them with kindness, he secured them as loyal supporters.
Fifteen miles in a day's march for Diodorus' source would seem the upper limit.
A bit wonky and overlarge but a table, as can be seen human capabilities push the time up and the distance down .