If your job description requires you to lift heavy weight, which it does, a lot of physical activity, which it does, and you have been told by the doc to not lift, or have physical activity that will aggravate your back, you will probably be found unfit. Every body is a little different when it comes to these injuries. Some people are severely hampered and some have little to no pain. If your injury kept you from deployment, then probably unfit.
The PEB may try to link your back trouble before enlistment to your current condition. I was trying to give you a heads up because they do that often with people who have not been in long.
You have some time before you get the results so research your options, know what your rights are, and do not sign anything that makes you uncomfortable without getting some advice. The PEB counselor may tell you that you have no options, but you do so know what they are.
They tried to tell me I could not use earned leave. Wrong, you can. In fact take your leave as it gives you more money than selling it back. You get TDY for job hunting and house hunting. You can file a claim with the VA as soon as you have a seperation date. You can request the seperation date, they do not get to tell you when it is. You sunmit a rewuest chit to PERS seperation department, and then they send a message back authorizing your date of Seperation. You may get severance pay if it is a service connected condition, which is what it should be since you hurt your back doing PT. But if you do you will have to pay the VA back before getting VA compensation.
Know your rights and let the PEB counselor know you do.