Some sets I have and plan to follow, adapted from 220 Triathlon
1) 50-50s: 50 x 50m reps getting you to 2.5km. Do 10 at either side for a warm-up and down. Mix up the strokes and drills in the middle eight sets.
2) Broken 1,500M: 15 x 100m.
3) 2 x 1km pyramids: After a warm-up, do a continuous swim of 25m hard, 25m easy, 50m hard, 50m easy, 75m hard, 75m easy and 100m hard, 100m easy. Rest. Repeat the 100m reps and go back down through 75s, 50s and 25s. A total of 1km. Repeat the set with a pull buoy.
4) Descending 50s - negative splits - starting slower, building up. Aim to swim 20 x 50m. For each set, include a rest time. The total time must decrease as the sets go on.