Good advice, but the up-front cost can seem pretty high. Running in more than one pair will help your biomechanics adapt ever so slightly to each shoe. These adaptations are a good thing because they help prevent overuse injuries. In addition, midsole foam requires as long as 24 hours to fully recover from a run. So if you run at night and then need to get an early workout in the next morning, a different pair will give your body more protection.