400m S-L questions

I guess that you will need to be careful in terms of volume (If you are adding these one-off 300m runs to your training week) and how this will effect your ability to get ready and progressively improve toward the nationals. You may also want to do some shorter speed work i.e. 60m sprints, as (based on what you have stated) 10 weeks seems quite a long time to go without specifically addressing this element.