Running Types and Intensities

E Easy Running 70% effort, usually 30 minutes to 1 hour in duration.
L Long Run 70 % effort for 1 to 2+ hours, not to be longer than 2 1/2 hours.  A long run is typically 25% of weekly total mileage.
T Threshold Runs

(intensity is 6 seconds per 400m slower than interval intensity -- see below)

There are two basic types of threshold running -- the first is a steady (approximately) 20 minute steady run.  Intensity is 88% effort (about 90% of max heart rate).   The steady 20 minute "tempo" run can vary by a couple minutes; for example if you have a three mile loop you run and Threshold pace is 6:20 per mile, you could be satisfied with a 3 mile run in 19:00.  Similarly, if threshold pace is 7:00 pace you might go 21:00 for your tempo run.  Better runners may well complete a 4 mile distance if threshold pace is near 5:00 per mile.

The second type of Threshold work out is "Cruise Intervals", an interval workout done at Threshold pace -- same 88% effort used for a steady tempo run.  However, in the case of cruise intervals, a series of runs are performed with very short recovery periods between each.  An example would be a 5 x 1 mile at threshold pace with 1:00 rest between miles.  Or, 8 x 1000m at threshold pace with 1:00 recoveries.  A cruise interval work out would typically total 8-10% of total weekly mileage, at threshold intensity, in a single session.

I Interval Running

(intensity is approximately equal to 3k to 5k race pace; in any case, about the speed you could race for about 12-15 minutes)

98-100% of VO2 max or 98-100% of max heart rate.  A single session should involve up to 8% of total weekly mileage, performed at interval intensity.   Recovery time is about equal, in time to work time, i.e., if you do 1000m intervals in 3:00 then recovery time should be about 3:00 or less in duration -- jogging is best during recoveries in an interval session.  Individual workbouts in an interval session should be 30 seconds to 5 minutes in duration.  3 to 5 minutes workbouts are best, but there is a place for interval 400m or even 200m at times.
R Repetitions Several intensities are appropriate -- 1) 6 seconds per 400m faster than interval pace; 2) 3 seconds per 400m slower than season's goal, 1 mile race pace; 3) 3 seconds per 400m faster than current 1 mile race pace.  Number 1 is usually the slowest and is often used for repeat 800m.  Number 2 is often good for 400m and number 3, for repeat 200m; however, various individuals can use #1, #2, or #3 for all their reps.  Total quality in a single session should be up to 5% of weekly mileage.   Recovery time should be FULL recovery, possibly 4 times as much time in recovery as during work.  You are supposed to feel good during each part of repetition session.
Strides   Repeat 20 - 40 second runs at about R (repetition) pace, or about 'current' 1 mile race pace, with a couple minutes jog recovery after each stride.   Strides are best done as part of a warm-up of after a threshold workout or in the middle of an easy run.

Always warm up and cool down before and after all workouts.  Include some stretching after workouts and do some stomach, back, hamstring, and hip and thigh strengthening a couple times each week.