"such a long obligement upon the whole realm to your virtues"

It's from Areopagitica by Milton: "Neither is it in Godsesteeme the diminution of his glory, when honourable things are spoken of good men and worthy Magistrates; which if I now first should begin to doe, after so fair a progresse of your laudable deeds, and such a long obligement upon the whole Realme to your indefatigable virtues, I might be justly reckn'd among the tardiest, and the unwillingest of them that praise yee."

The text also has a footnote saying that "The Long Parliament met in November 1640, four years before Milton published this tract." What does that phrase mean?