I say that to say this; running is way better for my back than cycling. My left leg, mostly my foot, still haven't completely recovered from the nerve impingement, but I am running 30 to 40 miles a week with no problems (lots of trail running too, check my Strava :) )
I can't cycle like I used to on any "normal" bicycles. This position, bent at 90 degrees or more, that one is required to sit while cycling is actually pretty terrible for the lower back. Also, being male and being part of a large male dominated industry - firefighting, I see lots of male cancers. That is, testicular and prostate cancers. My department is participating in a massive testicular and prostate study; We are being asked to change our chonies often and wash hands before going to the bathroom as the scrotum is a sponge for anything it touches... smashing that region when cycling for extended periods of time seems like a bad idea (and it hurts).