Pilates is important for isthmic spondylolisthesis. Core strength is important as is avoiding much extension. Impact activities can be difficult in some people and modifications may need to be made in lifestyle choices. You might consider cycling instead of hiking/walking.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.