I have the Lowepro Minitrekker AW to take most of my gear when I'm away for a weekend but if I'm just out for a walk the bigma is always attatched to a monopod and either slung over my shoulder or I carry it with the lens mount. Don't worry you soon get used to the weight. I'd get yourself a pair of waterproof trousers though as carrying a seat might be a bit much. Always carry something waterproof that you can cover your camera and lens with in case it rains (but you already know that)
.
Just a small tip, get yourself a centre pinch 86mm lens cap, the one supplied is near impossible to get off when the hood is attatched, can be very frustrating.
