Have you talked to your doctor about your pain ? What do they recommend for exercise ? Because of your medical issues, you should really start with the recommendations of your doctor before the well meaning, but totally anonymous advice of people on the internet.
If you find it difficult to walk for 15 minutes, can you walk for 10 minutes without any discomfort ? If so, then walk for as long as you can. A person can break their workouts up into shorter segments and still get all the benefits. Walking for 10 minutes three times a day still adds up to a 30 minute walk.
You could ask your doctor if it's okay to swim. Being in the water can help take stress off our joints. Things you might not be able to do on land, you could do in the walk like prolonged walking.
However, you have to ask your doctor first. They know more about you than we do. Even if a member with a back problem replied with what they do, you have to keep in mind that your injury is not the same as theirs. that's why anything we recommend could be harmful. That's why if members give some advice, CLEAR it with your doctor first.
Your doctor might not want you to do any exercise. How does a person lose weight without exercise ? By watching what they eat. When it comes to weight loss what matters most is what we eat. Good nutrition is what takes the weight off and keeps it off. Exercise is what keeps our bodies fit and healthy.
You do the best you can.