Ted, the idea behind this particular test is to have it be as close as possible to the event to represent your actual swim fitness on race day. The swim offers a unique venue to do this, as you really can’t run a 13.1 miles time trial a week before the 70.3, but you can recover fast enough on the swim to do this.
It’s not a threshold test, as the distance of the test varies based on the event you are doing. It’s a calibration for your particular distance, and a calibration that is best done as close to the race as possible, and we feel about a week is best.
David