You might be able to remove yourself from the faction with the "removefromfaction" command. The problem is, I'm really not sure which faction that is. There are a few factions that mention "Imperial" in their description.
If you can find out, you might have a chance of reversing it.
Try it again, typed exactly as follows after clicking on a horse.
setav speedmult 150
No periods, no numbers other than the speed value, check for too many spaces.
Gotcha, worked fine. Thank you
An object already in-game, or an object spawned with the console?