Another great ride from Mike, this one is a 45 mile loop out to Walden pond and back. This loop utilizes the minuteman bike way which is a great ride, I have done something very similar but took the minuteman back again. Walden pond is a great bike destination as you can get a good ride in see some awesome sights, and not pay for parking!
View Esplanade to Minuteman bike trail in a larger map
If you feel comfortable riding off-road, and it isn’t the day after a major rainstorm, you can take the Reformatory Branch Rail Trail instead of Route 62 from Concord to Bedford:
http://www.bedforddepot.org/MinutemanBikeway.html#RBRT
http://www.bedforddepot.org/RBRT_guide.pdf