No, you dont have to start from scratch, im not sure how it's done at your club, but my best advice to you is to speak to your pathfinder director.
At my club, we accept persons who have passed the age requirements, to serve the club for 1 year, while gaining all the classes within that year, and do the master guide course the next year.
As an additional option, we also have a special class that teaches leadership skills, and then they will move on to the master guide course afterwards.