I live in Castle Rock.
Occasionally I commute to Centennial airport via Crowfoot Valley highway and Cherry Creek trail/470 trail. As a distance reference, this ranges from 20-25 miles depending on specific route. This is really the best N-S bicycle access between the metro area and CR-Franktown. Crowfoot has a wide, clean shoulder and good visibility. Unfortunately, there aren't many good choices when trying to connect Castle Rock to Highlands Ranch or Littleton. I will not ride highway 85 between CR and Chatfield. Too much traffic going much faster than the posted 50mph speed limit on too narrow of a road. Castle Pines has decent access to HR-Littleton but is not very well connected to CR either. You either need to brave I-25 (which isn't as bad as you might think), ride the narrowest portion of Hwy 85 and have access through the gated community at Happy Canyon, or risk a couple more miles of Hwy 85 and go over Daniels Park.
Hess Road, between Parker and Castle Pines is a nice ride, but I avoid it because I can't figure out how to make a loop from Castle Rock without getting on I-25 or Hwy 85.
Since my starting point is Castle Rock, my routes may be a little farther south and west than you are looking for, but there are a lot of really nice roads to ride in southern Douglas county. Russellville Rd (Elephant Rock 32 mile road coarse) makes a nice route. There are some good climbing loops between CR and Palmer Lake West of I-25 that vary in distance from 30-50+ miles. The Wolfensburger-Tomah loop seems to be the most popular with the local clubs, but I can't stand Wolfensburger. I prefer Tomah-Perry Park, Tomah-Fox Farm, or Tomah-Spruce Mtn.
If you are on a gravel grinder, there are several 25-30 mile loops that range from about 20-60% gravel roads. There are also a few gravel options to go between CR and CC trail/Parker Rd.
Castle Rock also boasts the Ridgeline trail system where you can ride 10+ miles of continuous singletrack without overlapping your own track, and the Rhyolite bike park has a semi-permanent cyclocross coarse, downhill pump track, and 2.5 mile singletrack.
If you would like some specific routes or maps, don't hesitate to ask.