Hello,
Sorry to hear about your troubles. It really is hard to offer full on advice as each fistula is unique and so is the surgery to fix them. The only way you may lose bowel control is if there is a portion of the muscle that is cut during surgery, Most surgons will try to work around this at all costs for that exact reason, but if the fistula tract goes through the muscle there is the increased risk.
My tract was high up and blind sided which formed the abscess I has a seton put in oct 2012 and I only took 5 full days off work and part time the following week. It was still tender after that but I managed. I didn't not changed my diet except for the first day out of the hospital and that was because I was too nauseas to eat. Depending on the tracts or your personal dealings with food, you may need to alter it. The first bowel movement after surgery is scary... but you body makes it happen... sitz baths for the first month is a real life safer in keeping things clean and healing. I would have one in the morning and one at night and after every bm if I was home. If not I brought wipes with me or a water bottle. It will drain for the first few months, many people including myself have found gauze has helped to trap leakage and if you fold a 4x4 gauze and tuck it, it will absorb the drainage and help prevent irritation. (this was after a couple weeks for myself as it will be tender for the first bit). I now work out, run , work, sit..with mine... I just don't swim and am grateful for the relief although I still hate cleaning up down there after a BM. Fibre mix does help to keep it moving..
Try to come up with a list of questions for the surgeon, including the risk, and then just one thing at a time afterwards. I hope you find some relief with the surgery even though it can be quite nerve racking.