Base training to training program

Alternatively, pick up the program with 12 weeks to go.  Many generic 16-20 week programs include a couple training blocks that are geared just to get people ready for the actual specific training needed for the race.  But, you should take a look at any plan and try to decide if it works well enough that way. 

Training 'down' to a plan from a higher base of fitness seems like a waste to me.  Though, perhaps not as unwise as training 'up' to a plan that one does not have the base to begin.  As you are in the former group, either look at altering the plan (in training load and/or duration) or look for another plan more appropriate to your starting point.

Unless you are facing injury issues, there is no reason to drop your training volume to fit a training plan.  Find a new plan, continue your current training until the volume matches your plan, or start the plan you have at the week that matches your current volume and then modify the end to continue building up to the race.  The more volume you do the better your race will be.  I maintain a base running mileage of 45-50 mpw.  I would never drop this mileage to fit a plan.  Use the plan for reference and then put in your mileage and create your own plan.  If you build 10% a week and use the plan's speed workouts you will do fine.

Good luck!