If you are going to run, I suggest starting with walking with ear plugs in. This way you can hear your foot plants. As you pick up speed, try to minimize the impact to your head with each foot plant. The right shoes and a smooth gait will be a big help. It will also be better for your knees and hips.

As others said, a marathon in the fall is very ambitious.

btw, Many never fully recover from the tinnitus. Do not use it as a hard and fast gauge or recovery. Head aches are the most likely symptoms to return with exercise.