i did two half marathons last year the nottingham half and then a smaller local one in december between september and the second one in december i proberly went out training about once a week. i still did a sub 2 hour time on both the second one was only slower than the first by about 8 mins but i had an ankle problem. In my opion u will not even get close to finishing with no training. If u think u could go out this week and run 10 miles if u can do 10 which at the end of the day its not even half of what your planing on doing. If your a fit person ie play a lot of sport ie football, rugby, hockey you should be able to do it IF you pace yourself but i almost be certain that you would not beat someone who has been training. thats my say only an opion but an informed one i am studying a level pe and iv run a lot in my relatively short life. joe