Running a 5K is simple:
Run or jog one-third of your weekly miles as a long run at 70 percent of your maximum heartrate, which means being able to talk and run at the same time. This modest pace stimulates your heart, lungs, muscles and circulatory system to adapt and become efficient absorbers and transporters of the oxygen and sugar which fuels your movement. Long runs improve your collateral circulation... you actually grow more capillaries.
You cannot grow more lung cells, but you can expand your lung capacity by running upright and proud instead of hunching over with the weight of the world upon your back. You can also train the muscles which make your lungs expand and increase their endurance powers just like you do the endurance of your running muscles.
Your oxidation enzymes increase with training and you'll stimulate more Myoglobin and mitochondria to bring in and use the oxygen.
Three or four days after your long run, run moderately fast. Fast running teaches you to run efficiently. Wasted movement slows you down and makes you more injury prone. Work on good running form. You will need a one to two mile warm up and stretching prior to your fast efforts.
Do not sprint. The simplest way to get your fast running is to find a quiet area with fairly even grass. Run efforts of 60 to 90 seconds. Run two efforts with a 20 second rest between them because it forces you to curtail your running speed. Then take a 2 to 3 minute rest with easy running before cruising a second pair of efforts or striders. You will run at close to 5-K race pace at first for these striders or Intervals. Don't take your heartrate to more than 95 percent of your maximum for this training or else it will be straining your system too much. After 10 of these sessions you can edge your intensity up a notch toward 98 percent of your maximum heartrate, which will still only be 15 seconds per mile faster than 5-K race pace. It will also be at 100 percent of your VO2 maximum. Running faster will not help you.
Every third week, run 5 to 10 minute efforts at 30 seconds per mile slower than 5K pace. This will save your legs from the harsh impact of fast running, and improve your aerobic ability. Actually, it pushes back your Anaerobic Threshold.
As the weeks go by, build up to 4 of the slow, but long repeats.
During your sessions at 5K pace, build up to 10 or 12 reps of 1 to 2 minutes. Use trails, packed dirt and tracks for variety.
At 30 miles per week, your training schedule would look like this:
Day one (usually Saturday): Jog easily for 5 miles, but include 6 to 8 striders of 60 to 90 seconds.
Day two (Sunday): 10 miles running at 65 to 75 percent of your maximum heartrate or about 120 to 150 beats per minute for most people. Then bike ride for 30 to 60 minutes.
Day three is a rest day or for very fit people, swimming and pool running or elliptical training for 30 minutes.
Day four: run 4 miles easy.
Day six: run 4 miles easy.
Day seven: rest
Run a 5K to 10K once a month for 6 months, then rest up for a few serious races at 5K.
Avoid sudden changes to your training. Add a mile each week to those long runs. Add a repetition or two to your speed running. Don't make your speed running speedwork. Fast running must be slow enough that it does not feel like work. Fast running is about learning to relax at good pace; you should not be sprinting at mile pace.