Hi meakie, As someone from a swimming backgroud, training for 400m swims is something I've done quite a lot. There's no need to do 1k swimming non-stop (arguably, this isn't even neccassary when training for 1500m either). I would do sets of up to 500 POSSIBLY 600 max at a time, but if I'm specifically training for the shorter distances (up to 400m) I will be training with sets of 50s or 100s. Do you have any idea of what time you want to do the swim in? and have you done a 400m before?