Shoulders

After key-framing the legs we work our way up to the shoulders. They need to be furthest out when the legs are extended out to their maximum. Let's start with the right shoulder. 

Start at frame 1 and select the shoulder control. Rotate X forward slightly to about 15units and Translate Z back to around -0.5. Notice the leg is slightly bent now, but we want it to be completely straight, so select the ankle control again and move it back until it's fully straight but still touching the ground.

Your first frame should look something like this. When we rotate the shoulders it allows the legs to stretch out much further giving more extreme poses. With the first frame done, shift select the shoulder and ankle control and middle mouse click on the last frame and press "S" because again, the first and last frames must be the exact same. So any changes to the first frame, you need to remember to do the same for the last. 

With the first shoulder key frame done we need to find the next frame where the shoulders would be furthest rotated and translated, which I think would be frame 8 as this is where the leg is highest and just about to kick out and down. So we'll do the opposite of what we did for frame 1, by rotating X back and translating Z forward a little.

Now we need to repeat the same process for the left shoulder, first scrubbing through the frames to see where the leg is furthest forward and back.

So for the left shoulder, at frame 5 the leg reaches it's maximum height so we can Rotate X back and Translate Z forward, only slightly like for the right shoulder as we want the shoulders to look natural.

And at frame 13, where the leg is stretching back do the opposite as before, of Rotating X forward and Translating Z back. As this gallop is an asymmetric cycle, the leg positions won't be the exact same on the right to the left so there's no need to worry about having the same units for both legs. Making each leg move slightly differently to each other will make it look less mechanical and give a much more realistic feel to the run.