I take it these 300’s are not extensive tempo? If so, why only 900m. If not,w hat about extensive tempo to develop aerobic energy system? Something like 3 x 3 x 300 at 75%. Seems to be a favorite on many boards.
Because the speed of 3x300m is 37-39 sec. That means very fast and hard effort.
in winter the 6x200m with 3 min rec and the easy runs inbetween the sprint sessions I consider to be enough aerobic conditioning.
I usually do 3x3x300m like you said in the very early beginning of the base training. Progressing to 6x200m after some months.
I really cant see the need of much more aerobic conditioning, when this will give me less energy for speed training and remember how anaerob an 400m is!