It's a best effort attempt. You're supposed to be out of puff at the end.
(Try fartlek/interval training though, 400m in 2 mins, 400m jog recovery, 400m in 1:45, 400m jog recovery, 400m in 1:30 etc reducing the time until you are maxing out on the 400m best effort. Takes about 30 mins per session and has been used as a training programme for people struggling to get their times down (or after long leave periods for conditioning before a fitness test).