I dont' know where we read it but DH said he read about this person that said he would go out and do no less then 90 minutes that is all he focused on initially. Speed and mileage come with time.....I wish I could go out and "tootle" around trust me Sunday when we had to take it easy with the little one in the trailer it was tough, because when I go out it's balls to the wall until I'm wasted and just crawl home because I only get to ride outside every so often but spin my hinder off the rest of the time.
I focus on my heart rate and time and I know it's embarassing but I average about 17 miles for every hour I ride, (without WIND).