I think that if you are planning on walking on to a d1 program you must begin to address your speed reserve this summer and no later. I think most d1 400m guys are capable of at least 22's. You can run until you puke all of next year working on your fitness but in the end, if you don't have the speed to run a fast 200 you won't be able to turn too many heads with your 400 times. I will repeat what I said earlier because I still believe it; fitness is not the main issue in the 400m and should therefore be trained secondarily. I believe you could patch up your fitness deficiencies while focusing on speed. I still really like the extended short to long plan proposed by marshall on page 1 of this thread.