Hi ~ It depends if you want to get off at Shinjuku station and walk to the start line or get off at Tocho-mae which is next to the start line (still have to walk a little inside the underground pathway). Here is the site for the train schedule, just enter Asakusa and Tocho-Mae. Hope this helps. Good luck on Sunday!
Tokyo Metro (one of the major sponsors) once again designed the Tokyo Marathon poster for this year. It is very nice! Their website also shows which line(s) to take to see the runners at different view points. Good job Tokyo Metro !!!