Hiking over New Army Pass from Horseshoe Meadow is never easy. Fortunately we spent the night at the trailhead, but one night didn't feel like it was nearly enough.
The hike started out easy enough. We followed the trail to Rock Creek. Shortly after crossing it we turned left, off the trail, along an old footpath. This is not a marked trail and you should bring a good, large scale map in order to navigate through this section.
We generally followed the river and returned to the trail near the boulder field before Long Lake. From Long Lake the trail climbs up to High Lake and then begins climbing towards the back of the cirque before beginning its switchbacking ascent towards the pass. The last time I hiked this trail was after spending two nights in Mammoth, one night at the Horseshoe Meadow trailhead, and another night at Long Lake. I was pretty well acclimated then. This time with only one night acclimating at Horseshoe Meadow the hike over the pass was considerably more difficult.
Once over the pass the descent on the other side becomes pretty steep. Eventually the trail levels out a bit as it nears Soldier Lake, then descends again for about a mile or so to Rock Creek.