Mix it up by actually running 300m and 400m repeats.    Try 3 x 400 meters, running hard, with 400 rest (slow jog) in between.  Then 3 x 300 meters, running hard, with 400 rest in between.  This will give you a feel for what a hard 400 feels like.  
Before this workout, jog a mile so that you are sweating.