If altered cats are supposed to get fat and lazy I'm going back to the vet.
I can hardly keep weight on my cats, they never stop running around the house as if there is a kitty nascar race going on and they eat eat eat and never seem to get any fatter.
They're not young either 10 and 8 actually (not talking about Orange cat or Fat cat as we so lovingly call him now, he has no problems with his weight and is a typical muscular fit cat with a bit of pudge)
I have no real suggestions or advice other than the great stuff that has been offered here already.
Courage is being scared to death and saddling up anyways. ~John Wayne