Recorded something similar to what your working on Greg and what I found to work great was a small platform 2X4 made of plywood framed by 2X6's.
Was able to get a mic under and then one up on top these provided for a great resonating of the steps/marching and sounded huge!
After a few tracks of the feet, added other strikes with a hiking stick, a metal dowel, tympani mallet, or just get creative on your own behalf. For these other items added the needed dimension to the sound much like a kick drum you want to get a click attack and the low/low mid end rumble. Really spruced it up.