Need exercises for bad back

[Mudcrutch]

I have had 3 back surgeries and need to lose 50 pounds!
I have 20 screws in my spine and this makes most exercises very difficult. I can walk fast for 1/2 hour at a time on most days. Any suggestions would be helpful!
Thank you.

[LaNeesPassion]

I would suggest a lot of stretching which includes hamstring stretching. The hamstring is a very large muscle that often times affects our back. By stretching this will help with some of your back pain. I would suggest that you do core strengthening exercises which will also help strengthen your back. Core exercises such as different variations of planks. 

[Purplepower]

I've had 1 low back surgery, and 1 neck fusion. I have fake discs in my low back and neck, and a plate with screws in my neck. I have nerve damage in my right leg especially my right foot. Low back was in 2001, neck in 2014. Here's what I do.
Water is extremely important. Our discs are made of water. One of the best exercise/stretches I do is the superman. Lie facedown on the floor, lift your arms and legs off the floor and hold. As you do this you will be able to hold longer and lift a little higher. Do this just a couple times to start and increase as you get stronger. Variations are lifting only one arm and the opposite leg at the same time. Another good one is broken table. Get on hands and knees. Now extend one arm straight in front of you and the opposite leg goes straight back. Hold a few seconds and alternate. A third is lie on your back with legs flat on the floor. Bring your knees into your chest and hold your knees with your hands, pulling them into your chest. Hold and release. Repeat and gradually increase time you hold. Variations are to only do one leg at a time and have the other straight out. All of these will help your support muscles. Once you do those a bit, add in lots of core exercises. For those of us with bad spines, it is important to keep our cores strong. I also do specific neck exercises/stretches. The more you add these the less pain the less chance for reinjury etc. God bless you. Hope it helps.

[Ukase]

Here's my take: 

 You have screws in your back. That's significant. I would be extremely wary of any advice you got from anyone on the internet, unless you knew with certainty that they knew what they were talking about. 

I recommend you consult a bio-mechanist, someone who understands how our bodies are supposed to move. You'll need to do some transitioning exercises, most likely, before you can do anything serious. 

Also - I wouldn't walk fast - just take it easy. Movement is movement. As long as you're not sitting all the time. 

Most of our caloric expenditure comes from breathing, blood pumping, digestion - things we don't even think about. If I were you, I would write down everything I ate. You can exercise later. For now, just move. 

Determine your baseline metabolic rate. Don't consume too many calories, and the weight will drop off. You'll be quite surprised. When we exercise, we get hungry and can sometimes fuel that exercise more than we would if we'd never exercised at all. For example: if I don't walk - I can keep my intake to 1800 calories a day, easy. (while consuming 2400). But, if I do walk - and burn 4000 calories in a day - I end up consuming 3500! Which tells me, I should have never walked in the first place - from a sheer math perspective. Every movement has a biological tax.