Not knowing the road conditions, but having ridden on some pretty rough roads (read ungraded corrugations) for days at time with a sprung Brooks saddle I personally wouldn't bother with a Thudbuster. In fact I am inclined to go back to a Brooks B17, i.e., without the springs.

Lowering the tyre pressure helped me I feel.

In terms of distance ridden daily I am agree with the previous comment. I think you need to be more realistic as to what you can achieve. Again this is personal but I work around 75 km to a 100 km with a the 100 km being a "big day."