Won't get too much benefit from 400ms i dont think. VO2Max doesn't get stimulated for the first 2 minutes, so if you run a 400m, you are only stimulating it for 45-60 seconds. Keep doing 800ms. Also you are running them too fast, if your 5k pace is 6:15 per 800m then you have no business running an 800m in 5 minutes, you aren't stressing the right systems and begging for injury. Instead, run 3-4 800ms in 6 minutes each with 6 minutes rest inbetween. Don't do this workout more than once per week.

Also, add 8x100m w/ 20 seconds rest to the end of 2 of your easy runs per week for that basic speed.

What's your mileage like now? Do you do a long run?