I was in the same situation last weekend on the BCM600.
I normally do 200 / 300 with just a small seat pack, containing 1 spare tube, levers, repair kit, multi-tool ans couple of quick-links.
For a 600, I fitted a topeak seatpost-clamp rack with a slide-on bag.
I carried all the usual stuff, + 1 extra tube.
I carried an extra layer of fleece, in case it got cold.
My waterproofs went in there, too, to get them out of my jersey pocket.
Some extra rations went in the bag too.
There was spare room in case I needed to remove a layer.
Oh, and also a 50Kg Lead-Acid battery to satiate the Garmin's Hungaar.
In my jersey, I carried extra bonk rations.
I used the bag-drop facility ( I think I won that, I didn't spot anything bigger than my old purple Karrimor Hot Ice day-sack, containing 2 whole changes of kit! Prepared for a soaking all the way round, I had fresh kit for the evening ride, and fresh kit for the Sunday 200k. )