A 5K race provides an excellent opportunity for running newcomers to get their feet wet. However, because the distance isn't as daunting as other races, people tend to make lots of mistakes in their training. Here are the four biggest mistakes made when training for a 5K. (See 12-Week 5K Training Program.)
People seem to have the misconception that to get better at running, they need to run. Not true. Running is an extremely repetitive activity, and if your body isn't strong enough to sustain the repetition, eventually your movement patterns will break down and your form and efficiency will suffer. If you're training to run faster and longer, you must lift weights. (Are You Soft or Ripped? Try This Program to Build Muscle.)
Again, if all you do is run during your 5K training, you're going to run yourself into the ground. Try peppering different activities into your training routine. Swimming or spinning are great options. Not only will they help you recover more efficiently (because they're non-impact), you won't plateau in your running, which is every racer's fear.
If speed is your goal, running three miles repeatedly won't cut it. Speed workouts are absolutely necessary. Hill sprints, half mile and mile repeats and fartlek runs should all be part of your training plan.
Recovery is crucial because training breaks the body down. Recovery builds it back up, making it stronger and faster than it was before. If you're not giving your body enough time to recover, you will hit a plateau, overreach, sustain an injury, or overtrain. It's just a matter of time. Program active recovery sessions into your training plan. Use low intensity flushes, foam rolling, stretching, and cold therapy. (Check out Your Guide to Optimal Recovery.)
Training for a race and losing weight do not go well together. When training for a race, you should be fueling for performance, not for weight loss. If you deprive your body of the proper nutrients and calories, your training performance will suffer and you won't be ready on race day. If you need to lose weight for a race, train for weight loss prior to beginning your race training.