Walk around the cabin every few hours if it’s a long haul flight. For a 5 hour flight, maybe get up once or twice. Get an aisle seat so you won’t disturb anyone when you get up. You’ll also have a litte extra knee space towards the aisle. Bring along ergonomic support like pillows for your neck or lumbar. If you’re really hurting bad, you might have an underlying problem that your doctor can help you diagnose.