Hi Jo-
Glad to hear you are on your way to recovery. I just wanted to share a couple of thoughts.
I had fusion surgery on my neck 1 month ago. I tried everything to avoid it as I was so afraid it would be the beginning of the end, but my nephew who is a fire fighter had me speak to two of his fellow workers who had had low back fusion surgery and they are both back to doing their jobs fully, water skiing, weight lifting, surfing, etc with no problems. That helped encourage me to go ahead and I am sooo glad I did. The debilitating pain and loss of use of my right arm are gone and I am already back to riding with Dr.'s permission (he said everything is so well screwed together he doesn't think I can hurt it!). Granted, this is neck and not low back so my recovery is faster, but I think you will be fine too. I did a lot of research on recovery time before I did the surgery and found that the usual recovery time before being allowed to ride was 3 months for neck surgery and 6 months for low back.
As others mentioned before, the firemen warned me that I would always have to keep up on exercise to keep core muscles strengthened for support. Learn proper lifting and bending techniques to avoid injury.
One more suggestion I found on the web was to get a gel seat for my saddle to help absorb some of the trot bounce. I started out riding at the walk after having someone else warm my horse up for me. Then I did just a little trotting, slow jog, moved up to posting the faster trot and went on to loping. I am a little sore after riding, so I only ride every other day or so to allow time to recover. I hope you are back into the saddle before long, but do take your time to heal!