Re: Exercise to help fusion?
The most important thing to remember is that it takes a long time to recover from spinal fusion. You are talking about months, not weeks, and it takes a good year for the new bone to set up and grow strong and hard.
The best thing you can do right now is to take many short walks each day. This primary purpose for this is to keep the spinal nerves stretched out. This helps to prevent scar tissue from attaching as it forms and fills in. It is much better to take frequent, short walks than one long walk...for a variety of reasons. It is important to not get too tired. It is important that you not aggravate the sciatic nerve from too much activity or improper activity.
There is nothing you can do to hurry along the healing process. The cells will grow in at their own speed and no amount of exercise or food or anything else is going to hurry this process. You can keep it from happening though by not resting sufficiently, by smoking, by not eating proper nutrients, etc.
Regarding sitting: you should not sit for more than about 15-20 minutes at the beginning. Sitting puts 30% more stress on the discs than standing or lying down. Even later in recovery when you can sit for a longer period, it is important to get up and walk around about every hour or so.
Try to spend your time walking, lying down or reclining with your back well-supported with a variety of pillows propping you up in appropriate places.
You may not start to feel better for several weeks -- it really varies a lot. I was almost comatose for the first eight weeks with my first fusion. With a subsequent 3 level revision, I felt great by the 12th day...so there really is no rhyme or reason to the process from what I can tell.
Just learn to be patient and take each day as it comes.