The 6/1 fartlek is a moderate effort. Really it can be a 5/1, 6/1, 7/1, 8/1 whatever. The idea is that you run 1minute hard every mile or so. In order to make things easier, i.e. you don't need to keep track of mile marks etc.. you simply do a hard minute on every 6th minute, or whatever whole number is nearest to your regular pace.
So on a half hour or so run you would go hard starting at the 6th, 12th, 18th, 24th, 30th minute marks. In between you run at the fast end of your normal training pace. This is a GREAT way to mix in faster running when you don't have a lot of time or motivation for regular workouts. It is great for early in a season or when coming back from injury and your not quite ready for full workouts. To be honest you can do this run every day if you want and just get a touch of speed all the time.
So often we get in a rut of just running and not getting some faster running in. This workout is a great way of sneaking in a little speed when you aren't physically or emotionally up for a bigger effort. I use this sometimes after a crazy day at work when I am whimping out on a scheduled workout but rather than just slogging along at normal pace I can do one of these and the effort isn't that much different from a regular run but at the end of my regular 20k run I have done 12minutes of fast running so between 2 and 3 miles of quality work. Doing it once isn't a big deal but over time it can make a big difference.
I also love this one when I am traveling because you don't need to know how long the loop your doing is or how far you are going, or even where you are going. Just set the timer on your watch and start running.
The last great thing about this session is that if you are having a good day you can start hammering the rest minutes and it becomes a tempo with hammer intervals. Often I set out doing this because I'm lacking motivation and before you know it I'm running 5:20 to 5:30 pace on the recovery stretches and a salvage day becomes a great day.
So the next time you find yourself heading out for a run instead of the workout you should be doing try the 6/1 and you will at worst be a bit better off and at best save the day!