(starting pattern on the right, and resulting pattern along path on the left)

I just want a single path of foot prints along the line, like in the vine example of this guide.
To get the foot prints pattern, I opened some png files in Inkscape, traced the bitmaps, selected the two feet in the position they're in, then used the objects to pattern option. I don't know if that last part is what's messing it up or not, but if I don't do that, it gives me an error box when it attempts to do the pattern along path thing.