8 minutes for 400m of breaststroke would be as fast as I've ever seen anyone who didn't have the name of a country on their suit swim that distance. I hope you're able to pull it off. I don't know much about distance running and bike riding, but I know that sometimes I like to swim 50 yards of breaststroke because I get to breathe more often and I don't feel as exhausted after a lap. However, if I try and swim all breaststroke, when I'm done, my arms ache like I had a burnout workout with weights.
Again, congrats.