Apologies for missing your answers to the earlier questions, maybe we were posting at the same time? Could just be me being slightly toasty. Some good advice above. IMO shorter intervals (2-3 min) can help from getting dropped somewhat by just developing a higher threshold for swimming in lactate, but also from any increases in fitness (or more appropriately stated increased efficiency via those energy pathways). The old timers used to call it "speedwork" for a reason. Personally I'd get on a steady diet twice weekly for a few weeks before the next race - 4 or 5 repetitions with double the time resting spent working making sure that quality is consistent through the entire set (i.e. if not able to complete the goal either went to hard to early or too many reps in the workout but that's really oversimplifying things). Over/Unders are also a good way to duplicate the demands during a race - going slightly above threshold for a couple minutes and then, unlike a traditional interval with resting by coming fully off the gas, recovering at slightly below threshold instead.