nutrition for long distance swimmers

First I will introduce myself, for I am new here. I am from Poland and practice for long distance swimming. I want to participate in masters competition in winter (it is going to be my third time):) This morning when I was already after my workout the lifeguard came up to me and he started talking about training, nutrition itp. I told him that I am practicing for long distance, like 400 and 800 and he gave me some nutrition tips. He said that long distance swimmers need mostly fat and little carbs, because their effort is aerobic. Here I was quite puzzled. I know that you burn fat in case of prolonged effort at low intensity level. But this is not what the long distance swimmer is doing! When I do my 2x800 or 4x400m sets I try to do it at relatively good pace (though, of course, not at the sprint level ). And if I have had a meal rich in fat and low in carbs I feel week during workout. Especially when my volume is 3.500-4000m per session. What do you think about it?