Having no personal experience with the breast engorgement issue I can't comment on pain/leakage etc. But I would think the idea of pumping during T2 would be good. A fast HIM is under 6 hours with some people coming in at the 4.5 mark (those are pros). A lot of people come in up to 8 hours. The run is going to be the tough part which is why I suggest a pump at T2.
Secondly, while you are breastfeeding your hormonal levels are still such that your ligament stability is impaired and you are much more likely to have problems with your joints, especially your pelvic joints, during the run. If you still want to race, you should see if you can get a SI Belt to stabilize these joints while you run. Not only this, the connective tissue is lax and you will not have as much support for your uterus and this could predispose you to problems like prolapse.
I understand your desire to do this race and the swim and ride are not really an issue. I also admire your ability to stay in shape during your pregnancy. But if you were my patient, I would strongly advise against doing a 13.1 mile run this soon after delivery and while still breast feeding.
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