Battalion Recon Marine?

Hello! Basically I've been trying to figure out what would be a good weight for someone my height undergoing the Basic Recon Course. Right now I'm 5,7 and weight 160 pounds. I'm overly muscular to some extent. Back when I was lean and 140 pounds, I was running 18:25 minute 3 miles, but now I feel much more cumbersome, running at about 19:30 pace for 3 miles. Is this satisfactory? I mean a lot of the BRC is rucking, and being short it would most indeed help to have a huge muscle index, but at the cost of a little bit of my swim and run times. What do you guys think? Is a 6:30 mile pace for 3 miles really bad? I mean, most guys are necessary olympic athletes, but I imagine that they are probably gazelles. Is a 19:30 5k fine and all that matters is my mental toughness?