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Does this pain seem to get better immediately when sitting?
Do you find yourself leaning forward while walking for relief?
Well, the fact that you have pain that radiates in the groin, thighs, and this worsens with walking, this sounds like "neurogenic claudication" -- e.g. pain when walking because of "pinched" nerves in your lower back.
I would discuss this with your doctor, and see if physiotherapy would be helpful here.