The good thing about swimming is you can do hard interval work and have quick recovery and little chance of injury.
According to a lot of knowledgable people here it's recommended over endurance work because you burn calories yet maintain/improve musle mass.
5 min warm up
10min easy up, fast down
15min 50m sprints with 1min rest
10 min start easy, build up pace, push hard last 100m
5 min cooldown
Don't increase the total time.
Mix up the strokes, for instance up easy breastroke, fast crawl back.
Train 3-4 times a week like this and lower your base cals by 200/300 and see how your body changes, make adjustments there on in.
Don't do this first time round but in the future you can try first thing training on an empty stomach and black coffee.
Allegedly the muscle is not broken down, yet fat stores are utilised more. Not all have the balls for this though.