i disagree with all of the comments about how tricky, cook other things 1st, etc when it comes to brisket; it is a long cook in the lower temp range, but there is no "mystery" to it.
you've answered your own question, the marbled fat is what is going to keep your brisket (or ribs, butt, etc) from drying out and is going to give it incredible flavor.
there is a reason that prime-grade costs more.... marbled fat. Try to get your hands on the point muscle if you are able, my butcher will separate the point from the flat and sell to me. give it another try with a fattier piece of meat and you'll see.