Ok, I've poured over the manual, but I can't believe it. Am Sheagar walks way too slow. Is there running in this game and it's just some undocumented thing? I find it hard to believe anyone would want us to explore their game like this...

There's the speed spell. That's as close as you can get.

Ah, well, that will be fine. I can live with that. As long as it's mp requirements are low enough that I can cast it continuously.

I'm not 100% on this, but raising his dexterity *seemed* to increase his speed. I had one guy with high dex and one with low and the high-dex one seemed much faster to me at the time.

In the demo version available on Steam, you could run. Your character built up speed after walking a few steps; there wasn't any special 'run' key or toggle. This doesn't seem to happen in the GOG version...

I seem to be running about just fine in Arx Fatalis. The only problem I have right now is knowing whether I am actually sneaking or not. I press the shift key and all that but I just seem to be walking slower but I still get detected by enemies. That might have something to do with my stealth stat not being in the high levels yet.
In short, I have not noticed a running speed problem in Arx. Try raising your Dex/Agility stat and see if that helps.

You can't get speed until mid-game or something where you find the Movis rune, and it costs quite some mana. I'd say you can cast it reasonably often, but don't expect to be chucking lots of other spells at the same time.

**SPOILERS**
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There's a cheat spell using easter egg runes that permanently speeds up AS to a tolerable level. In spell-casting mode draw these three runes (attached): R A F
Am Sheagar will now move at a reasonable RPG clip.
For the full set of cheat runes go here: http://www.uhs-hints.com/uhsweb/hints/arxfatal/339.php