For an equally strenuous alternative with incredible geological sights, once you are in the bottom of Alamo canyon continue on to the Rio grande in a moderate off-trail segment.  Once within a thousand feet of the river turn left over the low hill to find the burro trail returning to the monument headquarters.  About 15 mi round trip and 1800 ft of gain, lets you see spectacular slot canyons, striated sandstone cliffs, and surprising stands of ponderosa and lush vegetation as you approach the river.  There are also obsidian stones throughout the Alamo stream bed.