You conflate having the muscle definition of a body builder and the necessary dietary habit with being fit enough to enter Phase 1/Basic training. Not only does one not need to do so, it can be counter-productive.
In the cookhouse, you'll be fed what you are fed; it will not include high-protein shakes, low-fat foodstuffs and creatin lotions and potions. One will not be working out in a high-spec high-tech gymnasium, clad in lycra and wearing the latest in Nike air-sole cushioned cross-trainers. Which makes me wonder if you ever went through Basic. The Army's aim is not to create physically perfect poster pinup specimens.
Its high-calorie cookhouse stodge, and good-old-fashioned beasting sessions in a gym with a few mats and ropes. And it starts from an assumed low-level of fitness, where the only expectation is for the recruit to put in maximum effort, gradually building up to a higher degree of practical fitness, not muscle definition and a diet-enhanced six-pack.
To the OP, don't panic, keep up a reasonably decent standard, and you'll be ahead of the pack. You'll still be expected to give max effort, be sweating like a the proverbial in a playground and spewing your ring up, and if you aren't you'll get shouted at some more...
****'s sake, let's all dig out D_L...
I look **** all like a bodybuilder... I'm 165lb and if anything, look like a footballer nowadays. I train for grunt like a squaddie or a footballer should. 4 days a week at circuits and half the class is girls, do a bit of running and when I'm in England, attend a class run by a Parachute Regiment PTI and a serving SBS man once a week.
None of it is hi tech... a step, a medicine ball, a barbell and a kettlebell, pretty much. Bags of legs, bags of sprints.
I use creatin for a bit of an edge, yes. But I eat real simple food - I have a bowl of smoked Coley and brown rice in front of me just now with a nob of butter in it. No need for protein shakes.
Army training's not all that. But whatever .. go ahead, pies and chips and do a bit of running - it will set you right up for army life. Don't forget to drink a lot while you're about it.