We can introduce this challenge to your workouts a little easier with the following three medley alternate sets:
Odd 100's swim: 25m fly (or tri-fly), 25m backstroke, 25m breaststroke, 25m free
Even 100's swim: 100m freestyle pull
Rest 20 sec
Repeat for 8x100m
(800m main set)
25m fly (or tri-fly), 50m freestyle
50m fly (or tri-fly) , 75m freestyle
75m fly (or tri-fly), 100m freestyle
Swim 4 sets of this 375m set. The first is fly (as written above), then complete doing back, breast and freestyle for a total of 1,500m.
8 x 250m as follows:
Odds are 250m are 25m tri-fly, 50m back, 75m breast, 100m freestyle
Evens are 250m freestyle
Rest 30s in between each 250m
Total distance: 2,000m
If you don't have time to commit to learning how to do the other strokes, then you can use the medley alternate sets above as alternatives to freestyle. Adding medley sets to your sessions will introduce great variety and interest to otherwise fairly mundane swim sessions. I promise you your time in the water will pass a lot more quickly as you have to pay attention during the changeover between strokes and focus on the techniques needed. The heart rate elevating qualities of medley sets will also give you a better workout overall, while adding in the benefit of reducing the amount of stress to your shoulders.