It is very important to keep the pace very easy and rhythmic. In a normal training run, you start easy and try to improve the pace. In over-distance runs you want to keep the effort the same and let the pace change based on how you feel and your heart rate. There are no penalties for walking and sometimes walking can actually improve your overall time by keeping you aerobic. You will want to have your own water or electrolyte drink on a waist pack.
Fluid replacement drinks like cytomax or powerade work well.
It is necessary to drink early and often because it takes 30 minutes to get fluids and energy replacements into your system. Don't wait until you feel you need it. On Sunday when you do your 3 miler, make sure to take it easy. You will feel very stiff at the first, so you will either start very slow or walking and you will be amazed how well your body will loosen up and you will get into a normal stride. Avoid sprinting at the end.