Too fast for tempo
Too slow for speed.
You have to set a workout in order to meet your goals, not to replace an existing session.
Work backward from the requirement of your event and do the calculation.
The first 200m of a 50sec 400m is supposed to be run in c. 24sec and the 2nd 200m in 26sec (FAT times).
I guess that if you only have 100m straight field, you need to run them little faster than 12.75 (hand times) and lengthen the rest. You can also do them in split runs format (e.g. 100m in 12.3 turn back 100m in 12.3, BREAK, and maybe 3 or 4 other sets)
Thoughts?