Practical Aerobic Intervals
This section gives a high level, practical overview of aerobic intervals. Aerobic Interval Training 101 gives some of the details around aerobic intervals and I would highly recommend the book Jack Daniel's Running Formula.
1 Find your target pace
The best solution to this is to use a race result. Go to VDOT Calculator and enter your race time and distance. This will give you a series of equivalent times for other distances. It will also give you a list of aerobic interval paces for various distances. So, if you did a 5K in 21:00, you would get 400m in 1:38, 800m in 3:35, 1200m in 4:54. Note: you must enter the time of a race you have completed, not your target time for a race. Please repeat out loud "I will enter the time of a race I've completed". More seriously, this is a very common mistake that you need to avoid. Train based on your current fitness, not the fitness you hope to achieve.
An alternative approach is to see how far you can run in 6 minutes and use that as the pace for your intervals. This is not so effective, but it can do at a pinch.
2 Choose a structure - short or long intervals
2.1 Short intervals
Personally, I like shorter intervals. I would suggest 400m (quarter mile) at the target pace. Recoveries should be about half the interval time. Start with 4 repeats, and build up over time to 8-16 depending on your goals.
2.2 Long Intervals
Use 800m (half mile) or 1200m (3/4 mile) intervals. Recoveries should be about the same time as the interval takes to run, which can often be accomplished by slowly running half the interval distance. Start with just 2 intervals and build up to 4-8 depending on your goals.
3 How often?
I would do interval training once or twice a week.
4 Ease into intervals
To reduce the risk of intervals, I would advise doing the first few training sessions at a pace slower than the 'correct' pace. This will give you a chance to adapt to the speed required. This is true for any type of interval training, as the stresses on the body can be much higher than the running you are used to.
5 Warm up
It is important to warm up for intervals. You can use an extra interval or two at a slower than target pace as part of the warm up.