This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
First off, keep in mind that the interval part of the run is the rest part.
This is where your body recovers and strengthens itself.
Secondly, say your goal is to run an 8 minute/mile 10k. Start your
intervals by doing 5X400m at a little under 2 minutes per rep. You'll see
that an 8min mile is a 2min 400, so to better that, you run a little
faster, as I said. Walk or jog between the rep (this is the interval).
Remember to keep with what you started at. If you jogged to rest, don't
walk during the next interval.
Intervals should be challenging, but not defeating. If you are having
problems maintaining your form during the course of the whole run, you are
doing too much. You should feel good at the end of your run, not ready to
Remember to warmup and cool down sufficiently before and after intervals.
10 minutes of jogging is suggested.
Other things to remember: you can customize intervals to achieve different
things. For example, to increase endurance, you can decrease your interval
while running the same rep. Or you can increase the rep and still do the
same interval. You can work on speed by running faster reps. There are
other variations as well, but I don't remember all of them.
Lastly, make sure you have a good aerobic base when you start, and don't do
too much too fast. You can tire your muscles out, and it will take a while
Your goal is to exercise your fast twitch muscles, those used for speed.
I've been doing intervals for about 2 months now, and it has made a
difference. The first race I ran after starting intervals, my time dropped
by about 15 seconds. I have a race tomorrow, and am hoping to improve on
that. I also notice I have more pep in my regular workouts. I get out
there, and once I'm warmed up, my body wants to run fast.