What Causes Numbness In The Shin While Running?

im 63 years old in excellent shape. swim 3 miles 3-4 times a week. up until a year ago I was running 3-5 miles 4 days week. I now have know reflex in my left leg. numbness in my shin and at times running down the outside of my leg but also on the inside of leg from groin area to my knee i get numbness and sharp pains. At times it feels like a tourniquet is around my thigh. I ve had two mri a ct scan and bone scan. I have herniated disks on L4 and 5 but the diagnosis can not confirmed that its a back problem because both sides should be hurting and my back is not bothering me. I started out getting check for a hernia which could not be found. No abnormal signs of arthritis of pelvic bone problem. everything looks normal there 3 epiderals in my back didn t help, 1 in my hip that helped somewhat. Had three doses of steroid 6 day pak that seemed to help the most. Any suggestions. I don t know what my next move should be.