From Inside Tri comes this nugget about training to swim at race pace:
If you have a hard time hitting your desired race pace during a given workout, try shortening your intervals until you can. For example, if your goal race pace is 1:20 per 100 meters, but you can’t hold that pace over 100 meters, try breaking up your speed work into shorter intervals with rest in between. In this case, I would suggest the athlete try 100’s at a 1:20 with 15-20 seconds of rest in between each interval. As the athlete grows stronger, he or she can add 25 or 50 meters to the length of each interval.
If my goal for an Olympic distance triathlon were to swim 1500 meters in 20 minutes (1:20 per 100 meters) and I currently could not swim a single 100 meter interval in 1:20 of swimming, I would be readjusting my goal. Maybe I don’t understand my own athletic potential.
On the other hand, even if I decided that 15 continuous repetitions for of 100 meters on 1:20 were a reasonable goal, I don’t understand how swimming 100s on 1:20 with 15-20 seconds rest is possible if I am not able to swim a single 100 on 1:20.