Hi Lindsay. I have family in Sedalia (next to Castle Rock), so I've faced this problem several times trying to map my way in order to visit.
First, making your way to Castle Rock from the TA (which routes through Pueblo): There are other possible solutions, but this is what I've done in the past: from Pueblo, hop on I-25 north (lots of traffic, but wide shoulder), ride about 25 miles and get off at Hwy. 83 (South Academy Drive). (Here you have two choices: turn right (east) and follow Hwy. 83 through CO Springs all the way up to Franktown and turn west to Castle Rock.) Or, turn left (east) and ride a mile or so and turn right (north) on Hwy 115. (This is what I've done in the past). This will lead you to the CO Springs bike trail system that will lead you all the way to the town of Palmer Lake and Hwy 105. (Here is a link to the bike trail maps that I stored online that will lead you step by step to Palmer Lake and Hwy 105: (
http://www.daystarbotanicals.com/biketour08/maps/ ). From Palmer Lake you will take scenic and rolling Hwy 105 north (S. Perry Park Rd.) for 10-miles or so and turn right on CR 46 (Wolfsenberger Rd.). This will take you right into Castle Rock--just a couple miles. This whole process seems more complicated than it is.
Castle Rock to Denver: From downtown Castle Rock, head west on SR 86 (Fifth St.). Take SR 86 about 6-miles to Franktown. Turn left (north) onto Hwy. 83. Take this all the way into Denver.
Denver to Ft. Collins: That's a tough call. I'd probably just grit my teeth and take Hwy. 287 to Ft. Collins. Sometimes that's what we have to do. Not scenic. Not peaceful back roads. But there should be shoulder most of the way.
This is how I exited the area in '09 in order to get on the northbound TA. It was very scenic. It was peaceful back roads. A couple hard-packed dirt roads (Colorado is famous for them). And, it was hilly (this is western Colorado we're talking about, so it's expected). It was a great option. (
http://www.bikewithamission.org/maps/overview.html ) Click on "Section two" and zoom in for detail.
Sorry for the long answer.
Have an awesome journey,
Ted
www.bikewithamission.org