B-B signifies a 'Beginner' level workout. You are a beginner runner if you've never run more than a few miles.
I-I signifies an 'Intermediate' level workout. You are an intermediate runner if you've run a 5K (3.1 miles) or a 10K (6.2 miles) previously.
A-A signifies an 'Advanced' level workout. You are an advanced runner is you've run a half marathon or marathon previously.
R-R signifies a 'RACE' level workout. This training level may be most appropriate for you if you're already in shape to 'complete' a half marathon and are looking to run a personal best for 13.1 miles.
Target half Marathon pace-This term is applicable ONLY TO RACE LEVEL RUNNERS. The idea is that you should have in mind what kind of average pace per mile you want to run for the half marathon (8:00/mile, 9:00/mile, etc.) When the schedule indicates 'X' number of miles at target half marathon pace, you want to aim to run CONTINUOUSLY the number of miles indicated at your 'target half marathon pace'. The remainder of the miles should be EASY.
Cross Train-Cross train days are a day of exercise other than walking or running. Examples are: cycling, elliptical training, or swimming. The idea is to
maintain your current level of fitness while allowing recovery from your previous run workout.
DAY OFF-Rest is a very important part of the training program. You will not improve faster by substituting additional runs in place of rest days.
'T' or 'Tempo'-TEMPO is a pace that is NOT a comfortable conversational
pace, but it is NOT a sprint either. In short, it's the kind of pace you might be able to maintain for a 5K or 10K.
'Fartlek'-Funny name aside, Fartlek is a pace slightly faster than ‘tempo’. The pace should be closer to your 5K race pace. Typical fartlek workouts are indicated as ’10 minutes fartlek’. What this actually means is you run 1 minute hard/1 minute easy for a TOTAL of ’10 minutes’. This is a bit different from tempo where you maintain pace CONSISTENTLY for the entire duration.