General question

First a little bit about myself, I was a swimmer since I was 4 and started competitively when I turned 10, attended Junior Olympics (placed 4th 3 consecutive years in a row in Mid-Atlnatic) and Eastern Zones up until I reached high school at which point I swam competitively for my high school which was in the highest division in PA, did districts and eventually states in my sophomore year, practiced as you would expect of any top level swimming program, year round, 6 days a week, over 5 thousand yards a practice (don't remember exact numbers). My dilemma now is that I have been out of the pool for about 3 years when I went off to university and did all the unhealthy things that a college student does. I am trying to get a beach lifeguarding job this summer and the tryouts are on June 11th (a week from today). I went to the pool today and could hardly get through a hundred at warm up pace without my arms just dogging out on me. My old coach whom I still keep in touch with told me that it was just because I have been out of the water for so long my muscles aren't used to that type of strenuous activity and that I should just keep swimming for the week to get back into it and I should be fine. The thing that was worrying me for the tryouts is that I have to do a 400 meter ocean swim in less then 10 minutes. Any competitive swimmer will tell you that that is a joke, but as per today at the pool, I was struggling to average 45 seconds a fifty at warm up pace. Is it simply, as my coach said, that I've been out of the pool too long, or is it that I am genuinely THAT out of shape that this beach lifeguarding will be a long shot. Thanks for your input.