I've never run a full marathon but have run a couple halfs and an 18 miler. I trained for my 2nd HM pretty much exclusively on the TM. IMO as long as you have run enough outside that you are used to running on pavement as opposed to a TM, I personally think it's possible to train for any race on a TM. Is it better to get outside? Sure, but I don't think it's mandatory as long as you have a goodly amount of previous outdoor running experience. It would also help if it's a distance you have run in the past as was the case with my 2nd HM. JMHO though.