Well, as Blue said, years of neglect.....but if you are just going to keep adding water instead of finding the leak, you are as much of the problem as the vehicle is.....
You are going to have to completely flush out the system, which means adding some type of coolant flush to the system, run the vehicle to normal operating temp, then let it cool down, remove the block drain plugs, disconnect radiator hoses, heater core hoses, and force water through the radiator, block and reverse flush the heater core, to remove all the rust/sludge in the system....
And all this will be for naught, if you don't find the leak and fix it....