Sitting back posture can be described as the pelvis excessively sitting behind the knee (specifically as mid-stance) - it will look as though you are in a mini squat. Due to this - the quads will become overloaded and this can put unnecessary strain on the knees.

Sitting back: at the start of every 0.25-0.5 miles - raise your arms overhead. This encourages the pelvis to shift forward. Hold this position for 5 seconds, and then bring your arms down to your side and swing your arms normally. Once you bring your arms down - keep in mind the idea to “think tall”, and this can help to encourage the pelvis to maintain its position.

Disclaimer: it is best to seek medical attention for a more individualized plan and approach to your needs for the best possible outcome.