lorilee
seeing your question on "is it better to train on a treadmill or outside for a 26.2 mile race", tells me right there you are 100% new to running.
my advice to you is train the proper way for a 5k race, and then try and improve your 5k times over the next few years..
leave the marathon to actual runners. most non runners, pick the longest race to enter, and get hurt training the improper way for it. seek out runners, get their advice and stick to a 5k. if you look at most marathons, in most cities, there are 20 guys out of the 5000 entrants that actually have a shot at winning the race. the others are just in the way..
good luck in your 5k training