WebMD explains that a good cardiovascular workout takes at least 20 minutes. Walking 3 miles at a moderate pace is better exercise, because it exceeds 20 minutes and produces the benefits of a sustained cardiovascular workout. Riding a bike for 3 miles can take less than 20 minutes.
The American Heart Association recommends a minimum of 150 minutes of cardiovascular exercise per week (roughly 30 minutes for five days). Walking 3 miles at moderate-intensity can last anywhere from 45-60 minutes. Riding a bike for 3 miles, however, lasts only 18-30 minutes. That's at least a 15 minute difference between the two workouts. By walking at a moderate pace for 3 miles, the heart rate stays elevated for a longer amount of time, and this provides a better overall workout.