what country do you live in? you describe a typical picture of disc protrusion and it sounds like it is resolving somewhat. Perhaps it would be helpful if you found a physio who can assess you correctly and advise on the correct/appropriate exercises

At this stage you may need some motor retraining (of the muscles) as the pain will have affected the way your muscles are firing and therefore not supporting or stabilising your spine well enough. this seems to be what is happening when you start walking over rough ground or downhill - possibly not sufficient stability leading to pain

I always warn my cients that they may think some of the exercises i prescribe for them are a bit woosy but they are needed to retrain the function (or even get them activating correctly) before strengthening and building endurance.

In my experience, Cortisone can be (1) helpful, (2) not make any change or (3) can make the problem worse. occasionally it "fixes " the pain for a signifiicant time but often the pain comes back if nothing else is done and repeated C injections have less effect and the relief period is shorter. there is some controversy as to how many one can have - one school says as many as make you comfortable, others say the minimum number due to side effects and/or damage to tissue. there have been promising studies done on animals where they have shown therapeutic ultrasound hastens healing whilst cortisone delays healing.

If a physio near you has an ATM2, this may be of benefit to you to - it is a machine whish fixes the pelvis and allows pain free movement.

A disc protrusion usually results from a damaged disc but significant studies show thta exercise can help if given and practiced correctly - good luck! If you have no pain, loss of reflexes or numbness you could hold off the surgery until you try a course of appropriate exercise