In all seriousnesss, this can be problematic. BMI data is a group statistic that is applied to individuals. A friend of mine, ex RN and RNR, had the unfortunate situation in life of being shaped like the 8 Ball off a pool table. A classic endomorph. Though fit enough for Naval Sports he struggled to get weight off his entire service career to fulfil BMI. Angry Doc is right, but in my case being of a similar build to you Andy, it's my left foot that's fallen to pieces. I would suggest, if you've not done so, to get decent trainers. Outside of Phase 1 Phys, where you'll have your Pusser's HiTecs on, a suitable pair matched to your build can miraculously improve your training ability. Choose a shop where you can be put on a running machine and have a gait analysis done. Us bigger units struggle on Bleep Tests because it's like turning round an ocean going tanker every 8 seconds. Getting decent footwear put about 6 shuttles onto my MSFT. All in the mind? Don't care, it's empirical...
I struggled with Phys all my regular career - it took me 6 months initially and a backclassing to get through BFT. The 1.5 miler time was the same for 16 year olds up to 32 year olds. I had to do it, and eventually I did, 2 stone lighter than when I'd walked through the gates.
You gotta make 28, you gotta make 28. Best of luck. Do it. Once you're in, it's a great career....