AutoCAD 2013 :: Creating 3D Model From File?

I have an AutoCAD drawing (I did not create it) of a proposed trail in my city, and I've been tasked with making some kind of model or render of a section of it. I'm at a loss for how to even start, unfortunately. In the end I would like to have an accurate 3D model of a boardwalk section of the trail with surrounding terrain. I have Sketchup, Google Earth, and Revit at my disposal.

My first problem is topography. When I import the CAD drawing to Revit it seems there is some topographical data, which will be needed to realistically build the boardwalk on top of. I was able to get the terrain of the area into Sketchup from Google Earth, but then I don't know exactly where to build the model.

I've attached an image of a floor I created in the outline of the boardwalk. The area in question begins at the two handicap parking stalls (represented as the two blue towers for some hilarious reason) and ends at the second square (a viewing platform). My preferred method would be get an accurate topograhic site in Revit, build the boardwalk, and then place surrounding vegetation so I can get some renders.

EDIT: I've attached "Screenshot 2" which shows I found I was able to create terrain in Revit by selecting a layer of the CAD file. However that layer is suspended above, and my placeholder floor is underneath. Now I just need to get the floor (representing the boardwalk) to go along the terrain.