This weeks session uses speed to maintain good form and tests you over 2 distances. Running shorter distances you should be able to keep higher speeds and at higher speeds your form is normally better than at lower speeds. You're aiming for a fast, but not flat out, run where you know your ground contact time is short and your bouncing along, not sinking into your ankle.
After the 400, take that form for another 400 but up the speed further for the second 400 in the 800m swim. 3 and 6 minutes should be sufficient for a full recovery between runs - if you feel you need slightly more recovery then take it. The important factor in this session is form.