I almost always use the following setup:
Large ThinkTank Airport Addicted for my main travel bag. It holds a full Z kit of cameras and lenses with accessories and my "spares and repairs" kit. It's a backpack that goes on the plane or in the car. Fully loaded, it's 30-35 pounds. What you can't see in this photo is the large pouches for accessories and spares and repairs, or the big laptop slot on the opposite side. This bag is discontinued, but there are other versions with and without wheels.
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For travel into the field, I carry my camera outside a bag with a lens mounted, and 2-3 lenses plus accessories in a ThinkTank Turnstyle 20. This is a light bag that has a very wide strap. It fits close to my body but is on my back and out of the way. It rotates around to the front for easy access and lens changes. The relatively small size and light weight means I can carry it for 5-7 miles without any problem. It's good for hopping from rock to rock, hiking, scrambling, or lens changes while in sandy or wet areas where you can't put down your bag. It comfortably holds a 14-30, 24-70, and 105mm macro. The F-mount 70-200 fit easily, but the Z-mount version is a little tight and fits better with just one other lens. I've also carried a single long lens - up to the 500 PF or 400 f/4.5 in this bag with another lens on the camera. On occasion, I've packed a flash ranging from the SB-300 to the SB-910 with a Magmod diffuser. There are nice pockets on this bag for maps or rectangular filters as well as for batteries, cards, tools, a small poncho, and accessories. I clip a Storm Jacket to the bag for rain protection if needed. When traveling, the bag is in my checked luggage filled with clothing or breakables. Note - this bag is my second copy. It is currently out of stock and might be discontinued.
https://www.thinktankphoto.com/products/turnstyle-20-v2
My thought process is that I don't usually need to carry a second camera because I'm close enough to the car or vehicle that I can go back and get what I need if necessary. It may be a mile walk - but it's an unlikely occurrence and worth the tradeoff. Filled with lenses, this bag is normally less than 6 pounds. The other thing is I am likely to use everything in this pack. I'm not carrying items that I might use 1 in 10 trips - it's items I use 30-50% of the time or higher. I use all of the lenses 80-90% of the time over a typical day. If I need a larger lens, I carry it without a bag on a Black Rapid strap with a RRS quick release. The long lens can range from a 400mm f/4.5 to a 600mm f/4 or 800mm PF f/6.3.