The Ascent (start line to top) is slightly more than a half-marathon -- 13.3 instead of 13.1 miles. So in order for the full Marathon to be an actual marathon, 26.2 miles, it needs to only come 12.9 miles back down. Thus, no 0.4 miles back down Manitou Ave.
Now why don't they make the Ascent an actual half-marathon? Can't answer that question.