The Leukemia/Lymphoma Society has been conducting its "Team in Training" since they were known as the "Leukemia Society of America". The quid pro quo offered by LLS has always been a strong training component which enables participants to become fit while they prepare for the charity marathon.
When I served LSA's Illinois Chapter, we had a program coordinator who had run in the Chicago Marathon organize our "Team in Training" and her expertise proved essential to maximizing participant satisfaction. You don't want unprepared runners getting sick or hurt during your charity run.
I bet someone at LLS would be glad to advise you on how to make your own marathon a healthy and rewarding experience.