Heres,the deal you will sign for an 11X contract and will be told either 11B or 11C during OSUT. The only difference is during the AIT portion 11B and C will have seperate training.
If you get a 11X Option 4 (Airborne or RIP) contract you will complete OSUT and then go straight to Airborne hold. Your physical will be completed at MEPS before signing the contract to see if you can qualify.
If you have doubts in your physical fitness to make it through and your have in your mind "good enough shape for basic" you need a severe atitude adjustment. Physical fitness in all aspects (endurance, stength and stamina) are a focul point of a career in Combat Arms. If you want to get Airborne or RIP, you need to drop the ding-dongs and cupcakes and start pt'in more. You are already failing just by having the "good enough" mindset because there is never a good enough in Combat Arms especially in two schools you just mentioned. Take it from a Infantryman that has spent thirteen years in service.