I have talked on here about a few types of alternations. If it isn't clear yet they are my favorite type of workout by far. They are the best of both world option that takes the best bits of tempo runs and intervals and meld them into a one amazing session. Perhaps more importantly though is you learn to recover at a fast pace. I don't know why but recovering at a quick pace is the one thing I have found that leads to the biggest improvements in fitness.
This specific workout is a great introduction to alternations. It is simple. It is adaptable and it is hugely effective. It can be done off the track but I would say it is easiest to complete it on the track. Like alterations sessions it is a continuous run. The main measure of the session is how fast your average pace is.
The how.. Run 400m at a faster interval pace. How fast can vary and we'll get to that. Then run 400m at a slightly slower pace. Keep this up until you can't go no more. Focus on two things. First on relaxing while running the fast rep. If you work to hard on the interval your day will be over very quickly. Next focus on being as smooth and relaxed as possible on your recovery 400m while still hitting your pace. Work too hard on the rest and again your day will be over very quickly.
The distance. Well in short as far as you can go. Now how far you try to go should depend on your goal race. If you are trying to get ready for a 5k you should only be doing 1 to 2 miles of these, rest included. Aiming for a 10k then only 3 to 4 miles of these. Half marathon 6 to 8 miles. Marathon 10 to 13 miles. Each time you aim to average around your goal race pace.
Now how much faster and slower than pace should your effort and recover be respectively. This depends on what kind of runner you are. If you are the kind of runner who can really fly on intervals easily you should start with faster reps and slower rests. You could have a range of 15 to 20 seconds. So let us say you were planning on a goal race pace of 6:00 per mile, 90 seconds per 400m. Then you could do reps as fast as 80 seconds and recover at 1:40. For runners with less basic speed you should try to have about 10 seconds between your fast and slow reps. I often refer to this session as 70/80. This is because I like to run my fast laps in 70 seconds and recoveries in 80 seconds. The tight range really forces you to recover at a very quick pace and this will enable you to recover and salvage races where you go out to fast but more importantly it will enable you to stay in control at a quick pace well into a race putting you in a position to make a long killing drive to the finish.
If you haven't tried this type of workout it is high time you did and this one is a great entry point because it isn't a mentally challenging as some of the other type of alterations.. If you have been doing them for a time then you should try this one in a few different varieties. You will love it!