Back Pain And Cortisone Injection.

Hi,
Last year some time in October/November I was working out in a gym and went thru some extreme physical training. Since then I was having tough time walking/running for continuously a mile or 1/2 a mile. Before that it was not like that(I used to swim and run a lot). And then I consulted the doctor,who recommended me to have an MRI done for my spine. They came back with the result that "L5/S1 :There is mild disc desiccation and a moderate right central disc protrusion with annular fissure measuring 5mm AP"
I tried the suggested physical therapy and back exercises, which did not help me much. Basically they told me that the rest is the best way to get back and then I really took some rest,by not straining myself much.
After that I thought it got better because I was able to walk/run for 2 miles per day continuously for 2 to 3 days and if I overdid, then it used to trouble me. So I thought it is really getting better,however when I run every day then the pain used to get in. So I realise this dependency, because of the
Muscle imbalances => , Disk Herniation happens=> Causes Inflammation => causes Pain.

Basically I thought, if I do some exercises I can strengthen those back muscles and move the disk back to the original position. But when I do the exercises it was paining,otherwise not. So I thought let me take the cortisone injection which reduces the inflammation completely so that I can start doing the exercises. But it did not work.
Once I had it(after my doctor prescribed it),for the last 2 weeks, it was extremely painful.The doctor suggested that this is definitely not for me. Now even if I take small strains, it is paining a lot. So definitely it has gotten worse and I was much much better before.
So now the pain is there even if I sit in some uncomfortable positions or if I walk for even very less amount of distance. I am taking mobic to reduce pain. What do you guys think the best course of action for me in my current situation. To start with,I want to reduce pain and get back to my life of swimming/walking/running. Please share your thoughts.
Eagerly awaiting your responses,
Thanks in advance.