When training for a specific goal such as rehabilitation or maintaining a healthy back/spin you need to take into account many different aspect as there are several reasons why you may be suffering from back pain.
Poor posture is caused by muscle imbalances - long weak muscles vs. short tight muscles. These could be caused by a number of things such as a poor exercise training program where you have excessive push exercises and not enough pulling exercises to balance them out.
Poor posture may be a result of your occupation, sitting too long at the desk or driving for a long time, as this all effects your muscle tone of shortening or lengthening them.
Muscles that aren’t functioning as they should or aren’t firing at all will cause the other muscles within the Kinetic chain to get over worked because they’re taking more of the load than they should or are designed to do. The glutes are a prime example of this with people that suffer with lower back pain, their glutes don’t fire at all so the hamstrings and lower back muscles get over worked and become short and tight causing pain.
So in this instance you have to re-educate the brain to start to use the glute muscles by completing activation exercises such as a glute bridge or hip extension.
Then when you feel able to progress go onto single leg.
Then progress from a stable surface to unstable and you can do this by using a swiss ball or Bosu.
These exercises will activate your glutes and posterior chain muscle, releasing the load from the over active muscles and in turn will start to ease the pain in the lower back.