How far out is the race? This will affect the mileage. If your client can run up to 7 miles/day then he/she is not far from be able to complete the 13.1 miles. Your client could at least once a week increase the mileage up to 9-10. At least once a week you client can work on speed too. Intervals on the track would be perfect for that. It’s up to you to decide the interval times and sets. You can also find programs online which you can use as a reference. Without knowing anything else about your client it is difficult to give you more specific suggestions.
One more thing. I know your area pretty well because I used to train clients there. If your client can incorporate some running on the sand it would be very beneficial. I know there are many open spaces where sand is available (at the lake). Also hills should be another surface your client can include into her running.
There really is so much more to providing something like this. Are there any issues to discuss right now? Aches and pains, etc. During the progression of volume there may be new issues popping up. How much time does the client have for trainng?
Is the client cross training? Type of training? How often? Training volume?
I am not trying to sound accusatory, but what we do is a science. Trainers should no longer be just putting together a “make it up as we go” plan. I have about twenty more start off questions in this type of situation that need to be answered before formulating a program design. And that design gets “tuned up” over the first two weeks to really fit the client and the goal(s).
You may be simply asking for some ideas to put into your clients program. So, don’t take offense. My tone here is intended to be one of curiosity and probing. And in that spirit, one of the training additions I like to have clients implement that seem similar to this client (currently accuston to half the distance of the target, apparently new to competitive running, etc.) is water running/jogging. This increases the volume of exercise for the muscle groups used, creates resistance throughout the range of motion, eliminates (in deep water) or reduces (in shallow water) impact stress/ground reaction forces which is probably the most dangerous part of increasing running distances with a client, and is actually theraputic for all of the tissues involved in running as well. I have worked with a few clients that were marathon athletes who could no longer sustain their road/hard surface training volume to be competitive without chronic injury/pain. Those that adhered to their aquatic training program were able to continue competing quite well.
I also train all clients (fitness to elite athlete) in ROM and foundation movements for all joints. This maintains function, can be very diagnostic, and can be progressed easily with elastic resistance (done anywhere/anytime) and cable machines when available.
If you would like to continue a dialog on this with me, contact me through my profile here in fitnessconnect. It would be much more effective we could ask and answer some questions between us.