1/2 mary... extra time in training schedule

I'm kicking around the idea of doing my first 1/2 marathon in September (not sure yet). I decided to go with Galloway's plan since it allows me to continue to stay very involved with CrossFit, since maintaining/building my strength is important to me.

I am more than capable of starting the plan now, but have 21 weeks until the race whereas the training plan is 17 weeks. What should I do with the extra 4 weeks? Speedwork or extra distance? The plan peaks at 15 miles and includes no dedicated speedwork (1st plan on this page: http://www.jeffgalloway.com/training/half_marathon.html).