fast days - How does prolonged (multiday) fasting work?

In Esther 4:16, Esther tells Mordecai to have the entire Jewish community fast for three straight days:

‘Go, gather together all the Jews that are present in Shushan, and fast ye for me, and neither eat nor drink three days, night or day; I also and my maidens will fast in like manner; and so will I go in unto the king, which is not according to the law; and if I perish, I perish.’

And in Jonah 3, there also seems to be a suggestion of some sort of prolonged fasting (though it's less explicit):

Now Nineveh was an exceeding great city, of three days’ journey. And Jonah began to enter into the city a day’s journey, and he proclaimed, and said: ‘Yet forty days, and Nineveh shall be overthrown.’ And the people of Nineveh believed G-d; and they proclaimed a fast, and […] And [the king] caused it to be proclaimed and published through Nineveh by the decree of the king and his nobles, saying: ‘Let neither man nor beast, herd nor flock, taste any thing; let them not feed, nor drink water; […] Who knoweth whether G-d will not turn and repent, and turn away from His fierce anger, that we perish not?’

How do such extended fasts work?

I've met many people who aren't able to completely fast for even 25 hours or so for Yom Kippur. Is it just a matter of motivation — that the people of Shushan and of Nineveh were more afraid during these events than we are on Yom Kippur? Or did Hashem lend them strength to continue fasting? Or something else?

Answer

I don't see (in my Lublin-style Mikraos G'dolos) any commentary note how many days the Nineveh folks fasted. I'd have to assume, absent other knowledge, that they fasted until the end of the day the fast was proclaimed, or maybe only the entire next day. (That's what I'd do if an announcement were made in the king's name that everyone must fast, with no duration specified.)

The Esther fast is of course harder to explain that way: it says it was to last "three days, night and day". But it was only three days: the Bavli, Y'vamos 121 amud 2, says that there was no miracle in their surviving this 72-hour fast.