I too am 70ish (a young one in my humble opinion) but can't carry a camera strap around my neck due to issues with a couple cervical vertebrae - a pay back from younger days playing sports. So, I use a shoulder harness that can handle two cameras. When I'm walking around, I'll use my Z9 and a 500PF (perhaps with a 1.4 TC) or the 100-400 S and the other Z9 will have a 70-200.
I have two harnesses. One is made by Black Rapid and works well, but always gets twisted up on me and the other, which is brand new, and still in the trial stage, is a leather harness made by Coiro. The Coiro is thicker and has padded shoulders and so far is extremely comfortable. The main benefit is all the weight is on my shoulders with no stress on my neck at all. I'd recommend looking into either or both. The Black Rapid I modified to use Magpul QD release connectors but after dropping the camera when the QD connector didn't hold I went to the Coiro which connects to the camera or the lens foot with a swivel connector and it also has a backup strap that connects to body of the camera. I've come to learn that Eric Bowles is correct when he says you really only need one camera and lens combination when walking about shooting birds. 90 - 95 percent of all my shots end up being taken with just a single camera. Nevertheless, the sling arrangement with just one camera still works great. So, that's my two cents, for what it's worth.