I would go with any of the Aerobic Base Builder ones. They run anywhere from 70-90 minutes. Another choice would be the virtual reality series where it's like you're riding along with Coach Troy. The autumn one is 45 min. The Lake Placid one is 3 hrs, but it can be broken down into sections that you can do at different times. The Arizona one is 2 different 1 hr UPHILL b*tt kickers. All of them are supposed to help build your aerobic base and endurance.