Plan on getting a BBL, but I am a mail carrier. Will walking me loose the fat I get transferred?

Walking by itself would not cause you to necessarily lose fat.  If you lose weight and body fat, then you would lose fat in the buttocks as well.  About 50% to 75% of the fat survives. Thus, results are permanent after the first 6 weeks or so.

Dear Dr.Harrydoll2019,
most patients will lose 30-50% of transferred fat after the surgery. Fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD

Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery