My first instinct was to delete the frames, and recreate them to make the run cycle. This did not work.
I tried again, trying to simply shorten the times between, and copy and paste it from there. This worked even less.
Finally, I was taught a different method. It was much more simple to take a half and half approach. Firstly, I selected all of the keyframes in the timeline, then brought up a move/scale window and changed the scale from 1 to a 0.5.
This shortened the loop itself, making it faster. Then, I took the loop, copied and pasted it so that it looped twice.
Finally, I went back over the animation itself, added and changed keyframes as would be needed to turn the walk cycle into a run cycle.