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As @kl says you will sweat a lot more at 30 vs 20 something, but at 5k in a pool id say the biggest factor is boredom, the whole getting out thing will provide a bit of a break and at least 50m you're not constantly turning.
Training wise, like any endurance event make sure you get a mixture of steady long distance swimming and more threshold type work to build strength. I presume that you haven't got ages to work too much on stroke improvements, but adding a few 100m of drills (catch up drills, finger dragging, etc.) into your sessions will make small improvements for you to improve efficiency and help alleviate the boredom!
Thanks for the advice. Yes it's in a pool so no wetsuit. I was also under the impression that water temp makes a big difference. For example I was regularly swimming 3 km odd at Gospel oak lido in the summer before work and what stopped me hitting 5k was fatigue. My arms were just exhausted and I felt I couldn't really continue. That's why I thought a sip of energy drink every km might help. Also the lido at the time was 24 which seemed perfect. I expect the pool to be closer to 30. How does that effect things?