You are probably not getting the same cardiovascular benefit from shorter rides simply due to the impact of the extended recovery on your anaerobic endurance. When we train, we try to simulate how we race in order to combat fatigue. If you are spending large portions of time recovering during those 24 miles (because you are commuting), then it is not exactly the same as riding those miles continuously.

But on the other hand, you are still burning 24 miles worth of calories, you are working your legs and lungs and you are conserving on the environment also! If you do long rides and interval training on the weekends, a century ride will be a piece of cake.