Hey guys, I know this is a really old post and I don’t know if you managed to fix the error you was having but I think I may have found the problem you had.
When you created the timeline and the added the curves did you make sure you changed the value as well as the time?
The first point (set at 0.0) should have the same value as the walk speed and the second one (set at 1.0) should have the same value as your sprint speed.
Hope this helps you and anyone who comes across this same error later on.