Here's two thoughts about carrying a heavy camera/lens combination.
1. A vest-like carrier such as the Cotton will ease the strain of a heavy camera/lens...
2. A monopod helps me in several ways... It takes the strain of the weight off my arms/shoulders while standing around. It balances the camera/lens when I am shooting and it can act as a walking stick when moving around. This is especially true if you are carrying the camera/lens using a Cotton Carrier.
Finally: a 4/3 outfit or even a bridge camera might give you the focal length you need without the weight of a full frame or even a crop camera outfit.