I have been playing with Revit to Trace and back today. As long as you
are using Revit MEP 2010 it may be worth the time. You need a good arch
model to start with. If there are a lot of interior voids you will have
a lot of walls that are not really their. There is a good white paper
out that helps in understanding.

Before I go much further, is this a complex job or relatively small? If
you still need help I may be able to assist.

Bob Vogt