Just start with 400m and keep doing it, ive just finished my lifeguard course and during the one week period my times have shot up due to the continuous swimming. Although i never had to swim 400m, are you doing the nplq?
On our course they said you have to be able to swim 400m, but we were never actually asked to do so, could be the same for you, its just used to filter out those who couldnt do it.
Also if your pool is the standard 25m then it'll be 16 lengths