Runner's World has good track training ideas. I hit the track once a week and vary the work-out. Ex#1-4-5 miles of a quick temp 200 then a jog 200; Ex#2- 5x1 mile at 10k pace; Ex#3- or a mixed routine of 1x 1600, 2 x800, 4x400 and if I'm not too tired some 200s. I try to get in a few 50-100 yard dashes weekly to help with a race kick.

It would certainly help to post more details about your current training so we can get a better sense of what can change. One of the most important things to realize is that your body will adapt to the training you give it, and that improvement comes from these adaptations. That said, if you've been on the same program for a while, you've probably reaped most of the benefits from your current program, and need to try different things: new workouts, a different schedule.

In response to your question, 15k is a pretty long distance. You could potentially reap benefits from simply increasing your training volume (more miles), or adding training at the other extreme (5k pace and faster). If you haven't before, I would try some workouts faster speeds (1 mile race pace). Good workouts for this include 12x400m w/ 60s jog recovery (at current mile pace), or four sets of 600-400-200 with 90-60 seconds of recovery and 2 minutes between sets. While these aren't race specific, training at higher intensities have been shown to develop running economy very effectively, so improving your mile time could lead to better half marathon performances. It is often said that Kenny Moore, fourth in the Olympic Marathon in 1972, trained like a miler with the exception of a long run (30 miles) once a week.

But just remember that all of this should be part of a well-balanced program, so what Kendall said is probably pretty reasonable.

What is your current training schedule like?

It looks like you are training for longer distances, so I would suggest that you gradually increase your mileage to 30-50 or more miles a week, running once or twice a day 4-7 days of the week. As you run more miles, your body adjusts to running. Build up your mileage gradually, increasing no more than 10 percent week over week. There are three types of run you should incorporate into your training schedule: the easy run, the long run, and the tempo/hill run. The tempo run can be on a track, as it is at a faster pace and can be measured by miles.

Easy Run: Most of your runs should be at an easy pace, where you are breathing easily and can keep up the run for many miles. This will form your base fitness level, and also serve as your recovery run.

Long Run: Once a week, include a long run. The long run should be at easy run pace. The distance of the long run will depend on the race you are training for.

Tempo/Hill run: Once a week, separate from the long run, include a tempo run or hill run. A tempo run is generally a 60 minute run divided into three parts: 20 minutes at easy pace, 20 minutes at threshold pace, and 20 minutes at easy pace. Threshold pace varies from person to person, but it should be a pace which you can keep up for 20 minutes without slowing down, but not an all out sprint. You may be breathing hard during the threshold pace, but you should have energy left for the last 20 minutes.

A hill run can be a run where you run uphill for 5-10 minutes at a fast pace, then jog slowly downhill. And repeat this 2-4 times. I've found running up a very steep hill for 0.5 miles, then downhill for 0.5 miles works best for me.

For track workouts, there is also the pyramid run we learned in high school. Run at a high speed 1x400m, 1x800m, 1x1200m, 1x1600m, 1x1200m, 1x800m, 1x400m, with half a minute walking recovery in between.