This past January, I ran my first indoor race. The Icebreaker Indoor Marathon in Milwaukee with one of my best finish times in over 10 years. By the way, all of my training for that particular race was on my treadmill.
If you live in the Midwest like me, you have been doing most if not all of your running on a treadmill these past couple of months. The end of March is giving us some pretty nice running weather, so it's time to head out!
In May, I will be running my first full marathon using my new marathon training philosophy: Less (approx. half) overall mileage with a focus on 3 or 4 hardcore runs per week (all at 75% + effort) and a strong emphasis on core-weights-functional training 2 or 3 times a week. And I've gotta say, my training runs have not been this good since I ran Chicago in 3:28 12 years ago. To say the least, I am very excited to find out the results of my new self-designed marathon training program.
Okay, back to the subject, running outdoors. Since most marathons are typically held outdoors on the street, it makes perfect sense that your body is well...used to running outdoors on the street! For me that will only be one day a week but for you it may be more. Keep in mind, the "shock of your foot hitting the ground with the force of 2.5 times your body weight. This is particularly important if you are a heel-striker, as many runners are, and run on unforgiving surfaces such as concrete. Repeated pounding in poorly cushioned shoes can lead to injuries, including to your knee and back."
When you head out for the first time, my best advice would be to take an easy run and get your body used to the pounding and if you can find a trail to run on, that might even be a better idea (that is what I will be doing). Also, it's not just practicing the actual running but also practicing using items like sunglasses, hat, gloves, ipod, waist pouch, squeeze bottle, nutritional packs, etc. that you think you may use on race day.
Funny thing about Spring Time Marathons in the Midwest, we could have snow, rain or 90 degree weather so you need to prepare yourself for every possibility!