If you just want to complete the distance I'm sure you will be fine with a plan, it's worth reading up on the route - I've not done it but understand it's a bit hilly, perhaps even aim to cycle part of it as part of your plan.
As you are in Cumbernauld I'm sure @
Pat "5mph" has already been in touch regarding her Belles on Bikes group - if not you might consider going out with them and/or join in on the CC Ecosse rides being mentioned in the Informal rides section - joining a club is also another good suggestion for upping your distance/speed.
Good luck! Regards, Mark