Good question.
The workout you speculate about is similar in design to ones by Pat Clohessy, coach of Rob De Castella, (famous elite marathoner); now retired. Deek would weekly run a workout in which he alternated 400s at 5k pace with 200s at a pace one minute per mile slower than his race pace (about 45 seconds for the 200). When fit, Deek would run 63s and 45s for a total of 5000m on the track. Now, is this a tempo run? By standard definition, I would say no, because the pace is well past threshold. I would call it a classic interval workout: controlled recoveries: exacted. Your 300s in 57 seconds is 5:04 pace per 1600m with 25 second 100s between, a 6:40 pace per 1600m. What really counts here is the pace relative to your fitness level. If your 300s represent 3k pace for you (which you mentioned), then I would say that after about 3 reps your workout is focusing on Max VO2 development, since the rests are short and there is really not much time for respiration to calm down. I would definitely not call the 300/100 workout a tempo run. Now, the real question is this: Is it a bad workout? I don't think so. It just serves a different purpose (max VO2 development). It has its place and time in your training plan, so don't throw it away. I do think that every runner benefits from blending different aerobic intensities into their training plan. If 57 per 300m represents your 3k pace, then, by my calculations, your LT to be 5:44 pace per mile (a short tempo pace of up to 20 minutes, per Jack Daniels' recommendation), 5:55 and 6:08 per mile for medium and long tempo runs (per my recommendation). I calculate your SDP (Standard Distance Pace) to be 7:07 per mile. You can do short intervals for Max VO2 development at 3k pace and longer intervals at 5k pace also. You can run 1500m pace short reps for power and economy improvement (Jack's term) at 71 per 400m or 35 per 200m. You can do anaerobic power at your current 300m sprint pace, but in shorter distances. When and how much you do of these intensities really depends upon which training block you are in. If your trying to develp aerobic endurance, then stay mostly in the long tempo and SDP range. If you have a solid aerobic endurance foundation, then do more work in the short tempo and Max VO2 range. If you are trying to work your speed a little, do up to 5 workouts at 300m speed, but not too close to your important race. Putting it together isn't rocket science, but it does require careful consideration of your realitic capabilities, strengths and weaknesses. If in doubt, and I am sure about runners doubting themselves, blend most of the key elements in cycles of 2-3 weeks and just keep repeating them, sort of like Deek did. From time to time, reflect on this statement: Speed without endurance is useless. I believe it is wise to error on the side of endurance. Remember, endurance can be improved 2000%. Strength can be improved 300%. Speed can be improved 10%. These are quotes from a book I purchased long ago called Advanced Running. Tinman