Easty,
I had fusion done on L1,L2 and L3 in 1992 (aged 35) and it was the best thing I ever did. After suffering severe sciatic pain for months.
It took me about 3 months to get back on my feet and active, doing all the things I wanted to do with a couple of exceptions. What must be understood mate is 'fusion' does in a small way affect your ability to bend (straighten) and twist at the same time. If I am sitting and drop something I find it easier to get out of the chair to pick it up rather than to bend and twist from the sitting position. Ten pin bowling leaves me stiff as.
The transition from paddling to standing may give you a bit of greif for a while but stay with it, mou may need a litre or two of the 'Amber Anti Inflammatory' after a session, but thats the price you have to pay.
Good Luck with your recovery and no matter how good the surf, take it easy for a while.