I had lower back fusion when I was 29 (now 31). I had stress fractures all around l5 and the bottom half of l4 had a good amount of damage. They took out l5 and part of l4, did a bone graft from l4 to s1, 2 rods and 6 screws.
I totally understand about hip pain, I have had 3 hip replacements done.
I would listen to your spine surgeon and get your back/core strong. I know that some ways of doing that are just not possible so you just need to be a bit creative on how you do some exercising. Keeping your body strong is a must do. I would do my very best to avoid back fusion.
After all the surgeries I have had, my hips hurt every single day as well as my back. Surgery is a 50/50, it can relieve some pain or it wont.
Also your lower back supports your upper body, having a fusion done will displace all that weight further up your back to your that is not meant to hold that much weight and that will cause even more problems later on in life with a good chance of having another fusion further up your back....its a horrible cycle and I am facing another back fusion.