What do you advise if i want to tone up but back and knee pain prevents me from doing high impact workouts. what are my options?

Core Exercises: Typically core strengthening exercises are the best to help relieve pressure and forces in the spine. I would try and google some or look them up on youtube to see descriptions. If you can do them yourself, wonderful otherwise ask your doctor to get a referral to a good physical therapist for further assistance. If your knees hurt that much, recommend getting imaging studies to see whats up.

Tone up: The best exercise is swimming which has no impact if done properly. Or you could do isometric exercises for all the muscles. These type of exercises can strengthen the muscle with minimal joint movement. Swimming in the ocean in Galveston should be fun !! all year long.. Good luck.