You should be fine if you are keeping your rectum. I have an ileoanal anastamosis (jpouch and no rectum) and do just fine. I eat over 30 grams of fiber a day and exercise like crazy with no problems. You may or may not have hydration issues. I only drink 20oz of coffee in the morning and then don't drink water until I go to the gym around 4pm. But we are all different so you will need to take care in learning what works best for you. You can meet others who have the IRA at www.j-pouch.org Most of the folks on that site had jpouch surgery for UC but we do have a few who had surgery for rectal cancer.
dx proctitis in 1987
dx UC in 1991, was stable until 1998
1998 started prednisone, asacol, pentasa, nortriptylene, ativan, 6MP, rowasa enemas and suppositories, hydrocortisone enemas, tried the SCD diet, being a vegetarian, omega 3s, flax, pranic healing, yoga, acupuncture, probiotics
2000 lost all my B-12 stores and became anemic
2001 opted for j-pouch surgery- now living life med-free