|Paavo & tower (Photo credit: HibaHaba)|
CT: Critical Threshold
These are the "easy" or restful running days, if you ever call a workout restful. These are the running days in between the tough days, and are meant to help your body recover, yet help you develop into a better runner. Critical threshold basically means bringing your heart rate and body up to a point under strain, but still in control, and you should be able to breathe easily and talk while running. CT days should be done 3 times a week on Mondays, Wednesdays and Fridays.
PPM: Pace Per Mile
These are the hard workouts of the week, but they should be done in a controlled way. Pace per mile means running set number as miles as fast as possible, but keeping the mile splits identical. At first, you will typically find your mile splits to be fairly inconsistant. Over time, try to bring the time splits together and then work on lowering your overall time. They should be done twice a week and usually on Tuesday and Thursday.
LD: Long Distance
This is the day to help you work on your slow twitch muscles in your legs and help you build up energy and strength for your longer races. Usually LD distance should be twice as long as your CT distance. LD should be done once a week on Saturday or Sunday.
Day of Rest
You can either take a day of rest, or do a low mileage CT workout on Saturday or Sunday. It is especially important when doing workouts early on to keep your milage low and give your muscles and joints plenty of recovery time.
For more info about the Paavo system, read through the Paavo forum on Runner's World.