"An Herculean task" is acceptable because "Herculean" can be accented on the second syllable, and there are a number of speakers that drop the /h/ and use "an" for words that begin with an /h/ and are not stressed on the first syllable. (I live in New England, and there are quite a few people who do that here. I believe it's a lot less common in other regions of the US.)
I have no idea why "an hundred" was used, or how it should be pronounced.
It used to be quite common, but it's fallen completely out of use today. However, in the last two or three centuries, it certainly has not been true that all words starting with /h/ got "an"; it has been "a horse" for the last three hundred years, while an hundred was quite common in 1800. (See Google Ngrams and consider Shakespeare's "my kingdom for a horse".)
Because there are several searchable versions of Shakespeare's works online, we can look at the words Shakespeare used "an" with.
He always used an with
Hebrew, heir, heretic, honest, honour, hour, humble, hypocrite.
I believe we can conclude that he pronounced these without an "h" (even though there is only one instance of an Hebrew and an hypocrite).
He used "an" once each with
happy, hand, hasty, heroical, household,
and for all of these, there were several usages with "a" (at least if you count other forms, like haste, hero, house). So I assume these were generally pronounced with an /h/, and the forms like an household are exceptions for some reason.
For hundred, "an hundred" appears nine times, and "a hundred" thirty-one. So something different seems to be going on with "an hundred". You can tell from the scansion that Shakespeare pronounced hundred with the accent on the first syllable. So why "an hundred"? I don't know.