Ease.
If your action doesn't have "Stride Length" use the Ease to repeat, crop, accellerate any action as much as you want.
I turned OFF "Has Stride Length" in thom's walk action here. The ease channel wont' appear if it is ON.
If your action has to have stride length (like a walk cycle) , then use the ease in the path constraint instead.
ActionEase.prj
actionEaseMP4.mov