I will often ride the DPRT from home (Long Grove) to the WI border and back with ~ 16-17mph average. I'll also jump on it in Lincolnshire (on days I have to go into the office in Bannockburn) and head up to the border and back. Nothing special, just lots of miles at speed with minimal road crossings so it is easy to keep the heart rate up. IMO the NW-MTB DG ride isn't worth it unless you are within a 10 mile radius... but it is worth noting that DG has probably the most narrow multi-track in the entire County which gives is less of a double-track feel.
I agree however I find that DG has the worst trail surface ever. I'd rather ride fox river trail which is also way closer