Like lesbentley, I always load the walk into the base layer and edit/adjust it using graphs, then use a new layer in additive mode.

I set a keyframe at frame 1 and last frame (straight spline) then move to the points where I want the change to begin, peak and end, set keyframes. All this without changing any poses, then go to the peak point and make the changes. This way all the fundamentals of the walk /run continue to operate, but modified by the new layer.

A good example of this in action would be a character running scared, twisting round from the abdomen looking back.

The begin , peak and end of action keyframes can be changed back to curved f it smooths the animation. It is just best to set an absolutely straight line to start with.

I seldom use Walk paths for the same reason as most other replies above, I think they could do with better editing tools. The other important point is whilst they move the figure round a curve, they do this by rotating the cycling figure, not by shortening the pace of whichever foot is on the inside of the curve. Delete the path and the figure will run in a straight line again.

Someone mentioned Nerd 3D's walk blends, I think he has a pack for sale at Daz, well worthwhile if you are using V4. He also gave a tip for older versions of Poser that there is a glitch at the point where the loop joins. His solution, simple but effective, delete keyframes 29,30,31   59,60,61 etc and allow poser to tween. If you want a perfect loop, make new keys at 2,30,31 do this simulataneously by selecting all 3 frames. Copy the new frame 31 and paste it at frame 1