Swimming Question, 400m in under 10 minutes possible?

Hi all my name is Joey and about a week from now I am taking a beach lifeguard test and the hardest thing by far is going to be the 400m swim in under ten minutes. I have been a pool lifeguard for four years and have had to do a 300m swim twice (one for initial testing and one for rectification). I have no benchmarks for time as the 300m pool swim I did was not timed and honestly I almost threw up both times after finishing it. I run competitively and my times are 17:15 for the 5k, 4:40 for the mile, and 55.xx for the 400m to give some perspective of my current fitness level. What can I do over the next several days to get ready for this test and how should I go about completing. Please not did any form of swimming since my pool lifeguard re certification test three years ago. Thanks!