As Mr Pogi mentioned above, tubing used for chain link fences works great as an antenna mast and its inexpensive. You could use a 5' section plugged into your 'J' mount, if its mounted in an area where there isn't too much wind exposure. I say that because the larger the antenna, the more it (potentially) becomes a kite. Receiving OTA can be a black art and you may have to raise or lower your antenna a foot or two to capture certain stations, so having a 5 foot mast gives you some flexibility to do that, if it proves to be necessary.
Check the existing coax carefully along its entire outdoor run to be sure no one (the installer) has not kinked it or damaged it in any other way. If you find damage, the outdoor portion may have to be replaced and if so, use black (not white) RG-6.
Most antennas do not have a built-in balun, so you will need one 300 ohm to 75 ohm balun. I use liquid silicone RTV (room temperature vulcanizing silicone rubber) to waterproof my coaxial fittings. I suggest using Scotch brand black electrical tape, not an off-brand, to secure the coax to the mast: I prefer tape over wire-ties because of the real chance of pinching or kinking the coax when using wire ties.