Eat less or do more. Unfortunately, it's that simple.
Exercise that you enjoy has to be the key. Swimming sounds a pretty good idea in your current climate, but watch that you don't dehydrate. Being surroungded by all that water can sometimes fool you.
40 lengths seems a good target- if it makes you sweaty, pant and a bit breathless, for 20-30 minutes, then you will be burning some calories.
I'd be a little cautious trying to reach your target immediately, 'specially if your a bit out of condition.
Try
2x25m
rest
4x25m
rest
6x25m
rest
4x25m
rest
2x25m
finish
for your first attempts, build it up as your stamina increases. (2-4-6-8-6-4-2), add a 10 in the middle if your up for it, go back to the original warm up on an off day.
This routine isn't built on science, only experience, but it seems to give you that post exercise buzz, without completely knackering you.
Avoid the post swim Chicken Tikka Masala washed down with Cobra beer though
