I'll try my best but i'm not at my PC currently so it's just from memory:)
After launching the divinity engine toolkit and loading the mod, just load a level (character creation is fine).
Somewhere on the top bar is a button that shows you root templates. Just press it and select "all".
At this point find the entries "MalePlayer" and "FemalePlayer" (maybe Male/FemaleCharacter? not sure).
Select each one and in the SideBar (activated from View->SideBar) modify the entries RunSpeed and WalkSpeed.
Save the mod and run the game. At save you might get some errors but it still should work.
