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.