First of all as a triathlete you have to do two other sports besides swimming…so the training in the pool can be used as non-impact training that will benefit your aerobic system and help you in the other two sports. So you may want to do yards/meters that seem above and beyond what you need to complete the 2.4 miles.
Secondly you want to not only finish the 2.4 miles, but you want to be relatively fresh after you get out of the water as well. This is different from training for swim only events where the goal is to finish the race as fast as possible, with no regard to how much it takes out of you. Having done alot of hard swim sets, especially when already fatigued, will help you with this.
Finally your race is 2.4 miles…the longest race in the pool is a 1650 I believe. The key sessions you will be doing will be alot different that what even a long distance pool swimmer would do. Main Set Examples…5 x1000 (best average); 20X200 (best average); 10X400 (best average); 4-6k long swim w first 2k bilateral, then build speed for the rest of the swim.