I have run races every week for months at a time. I try to pick a goal race every 4-6 weeks. I also like to mix up the distances and run 4 mile, 5 mile and 10k races. I sometimes skip a week of racing if the race is longer. As you get closer to the 15k, move up to racing a couple 10k's. You may also want to run a 15k to half marathon about 3 weeks before your goal 15k to get used to the distance and help your pacing.
You will find that after about 6-8 weeks of racing, you will peak and not be able to improve much on your 5k time, but you will be able to transition into longer races and run comparable times in those as the year progresses.
My week looked like this:
Sunday-10 miles steady run
Monday-hour stairmaster*
tuesday-3 warmup, tempo, 3 down
wednesday-hour stairmaster am, hour stairmaster pm
Thursday-intervals or easy 45 minutes if tired a.m., hour stairmaster pm
Friday-off
Saturday-5 warmup, race, 3 down am, hour stairmaster pm
*You can substitute easy running or eliptical for stairmaster.