Aerobic capacity, not speed, is usually the limiting factor in how fast you can race, even for a distance as short as the 5K.
The rest you take between repeats is as significant as the fast portions of an interval workout. Modifying the length and speed of your recoveries provides infinite training options.
More than half of all athletes will overdo it at least once in their running career. Use these tips to recognize subtle signs that you're approaching overtraining.