2005 Altima 2.5

I bought mine at 115k and had some minor fixes at the purchase. Now has nearly 123k on it. Winter is blessing me with all kinds of "odds & ends" to fix: Crank sensor, coils, plugs, battery cable, battery, and alternator. I have been aware of the cat converter issue. However, when sealing an exhaust leak, my mechanic said the one was replaced and the manifold was perfectly clear. (I am not consuming or smoking oil but i do have a valve cover oil leak that will be taken care of soon.) SO... what should I attribute my functioning converter to? Good luck? Is it possible the original owner had it replaced very early on (he was a Nissan dealer and it looks old)? Don't tell me to sell this car... tell me how to keep it running for many more miles. I would like to replace the cat but i am not seeing the urgency... funds getting low and weather getting colder.