If you can't work sprint into it, maybe make the thief's acceleration proportional to the mouse distance? i.e. if I want him to move slowly, I keep my mouse close to him, if I want him to go from a stop to full speed quickly, I twitch the mouse halfway across the screen... just an idea
Edit: Just played again, maybe my problem is that his slow speed is just too slow for my tastes. here's an idea - for every second spent moving slowly (below some very low threshold), he gains 1 "energy" point, which increases his max speed by a little. Then when you accelerate to full speed, every second spent above his base max speed takes away 2 energy points, dropping his max back to the base speed. This shouldn't add too much to your code, and would be a good effect I think. Of course there'd have to be a limit to the speed bonus, maybe +50% normal speed or something. Doing a graphic for this would be pretty easy too... just a rectangle that fills up and then empties out based on how many energy points you have.
But it's your game, so I'll let you do as you will