h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This is the true story of hell.

Once upon a time, before Thor and Odin entered the stage, the folks in scandinavia worshiped a godess named Hel (meaning "whole"). She was a kind of universal godess, the godess of life and death and everything in general. The word "Hel", by the way, is the oregin of many words: Holy (in swedish helig), hole (sw hål), healing (sw helande) and so on.
Later on Odin passed her down to the underworld to take care of the guys who died. What you got when you died was the "punishment of Hel" (sw "Hel's Vite", Hell in swedish is "Helvete") In those days, scandinavian and english languages had a lot of influense on each other. A lot of vikings vent to England, and our ancient godess hel's name is used worldwide.

It was my impression, based on norse mythology, that Hel was the daughter of Loki. I don't recall ever hearing anything about her being older than Odin, or being worshiped. What are your sources?

Many books are written on the subject. Hel was worshiped long before Thor and Odin entered the stage of scandinavian mythology. The center of the cult was in the very north of norway. People from all over scandinavia walked there to worship Hel.
I dont know from where the gods of the viking-era came, but it is known that when new religeons take over a region, the authorities from the erlier cults tend to get a new place in the new mytology.

Sad, but true. When the older God/desses were "absorbed" (if you would) by the ruling people's religion, usually in a way to make the populace accept the new religion easier, They were repositioned in a new place, which would invariably be of lesser importance than where They had been earlier. Thus how Hel went from being a worshipped and much loved Goddess to being meerly the daughter of Loki.