i think you should do it. it's good to push yourself physically and very empowering to know you can accomplish something like that. you get a huge boost the day of the run from the crowd and all the energy, so it's not like you have to hav e run 13 miles many times before to pull it off. if your body starts to hurt and you get injured, i'd say no. listen to those signs. but if you feel good and like you are getting stronger and are building up more endurance, i say go for it. you'll feel great having accomplished something that is tough like that.