No worries mate! I would keep trying to get those down, i got my 300 reps down to low 35's high 34's. Speed should be worked in during the week you should be doing a speed session in between all this longer stuff. also when you get your times down try doing som 1k reps for aerobic fitness! let me know how you go!
im 16, 63 kg and 5 ft 11 inches so a similar athlete to yourself in that respect i also specialize in 400m and 200m although more towards the 400m. it is important to know that the quicker your 200m is the quicker your 400m is as you should be able to run one of the 200m lengths of the 400 close to your pb for the 200, to put that in perspective i run the first 200 in 23 - 24 seconds. i am currently running 200m in 22.37 and 400 in 49.09. the key for 400m training is to do a balance of speed training ie. 60m, 120m, 150m. as well as endurance training such as 300m, 500m, 600m and 1000m. If you can do a balance it will improve your speed endurance which is key for both the 400 and the 200.
also you should look at michael johnson's videos on Youtube and he will give you some good ideas...
Also if you do 300m splits you should be doing them a lot quicker than 51 secs, say about ideally sub 40 but low 40's to start off.