Honestly it sounds like you may have other issues affecting your sex life. Back problems can screw up erections and so can mental problems. The fact that viagra does not help suggests a significant issue.
Id suggest trying to get healthy, physically and mentally, get your back in shape and get in good mental condition. You cant expect your sex life to be good if you are not healthy mentally and physically. Injecting a bunch of testosterone is not a good long term solution.
If you do suffer from low testosterone, there are natural supplements that can restore you to normal, or even above normal production. I take 2 supplements, DHEA at 25mg ED, and a natural supplement that lowers estrogen at half the recommended dose. When I get blood tests, my testosterone is in the 700-800 range, basically what a 20-25 year old man makes and I am 40. I am also very fit and eat very clean though. Although my libido is not as high as when I was 25, I get a very strong erection with no foreplay, just being in the mood and kissing/touching my wife and Im good to go.
i had to go to a urologist to get mine. they first will ck your levels then decide which treatment is best. i started with injections, then changed to implants(last longer than shots). i still have problems with erections, but was prescribed injections which i give myself an half hour before intercourse.
you may want to look to a neurologist ,, may be some nerve entrapment in your lumbar or something like that.