On a scale from the Riverlands during the Wot5K to the Shire, what was life actually like for the typical medieval peasant in say, England?

Highly depends on the time and place. During the Anarchy or other periods of chaos, it could get rather Riverlands-in-W0t5K-esque indeed. But if the harvest was good and taxes were low and there weren’t any wars, more like the latter. And often, it was somewhere in between.