First, MRI findings do not exist in a vacuum. You must convey your symptoms so that a comparison can be made to the MRI findings.
For example, you have a large herniated disc on the MRI (“At L4-L5 a left paracentral inferiorly migrated disc hernia is compressing the left L5 nerve root in the lateral recess”). Do you have left leg pain, weakness of the leg or antalgic posture ( a lean to or away from the left side?
As far as running, your doctors are correct that running increases the load on the lumbar discs. It is not a matter as to if you can run but if you are willing to take the risk that the added load on the spine can advance the degeneration of the lumbar discs.
There are less stressful spine loading cardiovascular activities that you can participate in (cycling, swimming to name a few) but I understand if you need to run and are willing to take the risk, then by all means continue to run.
Dr. Corenman
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.