If you’re spent after the swim you’re not gonna make it to the run, i push the pace that i feel i can sustain for the whole duration, so if i’m a 25min 1500m swimmer i’m not going to be able to last if i try to do a 21min swim. Comfortably uncomfortable.
Definitely spent by the end of the run though.
Aim for a PB but realistic based on training times and how you’re feeling on the day/what the course profile is.