Brad - there is an aquatics center in Manitou Springs located next to Cherry Park just down the road from the finish. As long as you show your race bib you have free access to the facility to shower.
In response to:
The website hints that showers are available in the park post-marathon. Has anyone used them? Are they decent? I will probably have to give up my hotel room before the race, so wanting to make sure I have somewhere to hose off and change.