Hi ~ It does take quite awhile before you see any bone growth after a fusion. It could be anywhere from 6 months to a year from what i've been told. Hopefully it won't be that long.
I don't doubt that you do feel weak. You've been thru a very traumatic surgery. It's going to be awhile before you feel your old 'self' again. But don't push too hard ~ you don't want to do something to the fusion!
I wish you the very best! I hope everything goes well for you and you heal fast. God bless. Hugs, Lee