L5through S1 eh that's a rough spot? Either way Guido had it right there man you need to see a specialist and., keep a tighter reign on your diet. Also when talking to your Primary Dr and your specialist look into AIGH ( Intra Articular Growth Hormone Injections). Yoga slowly at first until you build up a better range of motion in your back but again no serious exercise or training regimen should be undertaken with out consulting your Dr as spinal issues can be serious. Testosterone, and Deca may help but frankly the issue of pressure in and around the cord and Sciatic nerve may be aggravated by a long list of things that cause inflammation and a decrease in that inflammation needs to be taken care of first. Look into ultrasound treatments, as this allows for direct stimulation for soft tissues in the area ( it increases the availability of nutrient rich fluids by dilating blood vessels ect ect , and helping the tissue at the cellular level to eliminate fluids and take in new nutrient dense fluids.) Like myself and Guido said look into a specialist, and get with a PT that specializes in spinal issues. Eq is another good choice ( in the AAS range) as it shows an affinity for helping with bone density ( though many AAS compounds do )and increase in RBC counts and collagen synthesis. swimming would be a good thing less stress on the spine while providing resistance. That all aside; a nice lower impact exercise plan will help but you need to focus on functional movement exercises, and your range of motion other wise surgery may in fact be your last resort ( my mother has had 3 fusions in her lower spine a cage and 6 bolts trust me its not an easy surgery or recovery and from my personal experience with her recovery I consider it a last resort ) Though she has recovered and has made leaps and bounds in her physical recovery and while not back to 100% she is better then before the surgery- though again see a specialist and get set up on a routine tailored to your injury and pain level. Hopefully Guido can fill in some blanks with specifics for your injury.
Also animal mother, made a good point as long as you are operating at a caloric deficit you will loose weight, walking is great exercise and will help build strength across the board. But remember baby steps, especially coming back from a serious injury. I hope this helps as for the AIGH one last word there is a good bit of science behind targeted treatment with HGH, but its expensive and the science is not fully fleshed out. Just Google it and you will get a bunch of articles on it. Good luck brother and again I hope this helps and if there is any thing else we can do to help just ask brother. Cheers
