Running 3.1 miles -a stretch is quite an achievement. For you to perform your best, your body will need proper fueling.
In this case, you need balanced food - so as not to overeat and feel sluggish during your run.
Running on an empty stomach is highly unhealthy, and should never be practised.
Running is a high-endurance activity; thus, your running regime will be the same post and pre-training.