Several Audaxers find that a milk drink settles the stomach but, of course, YMMV.
My progression has been commuting to 13.5hr 200, to 14h50 300 (lots of drafting), to 9h40 200, to 8h25m 200 (lots of drafting). Got a 300 at the end of the month, then Seething 600 in July. My limited experience tells me...
1) using your commuter bike means no surprises wrt comfort
2) stopping (for food) any longer than is absolutely necessary provides no benefit to me. long stops make me feel worse.
3) riding with interesting people makes the time go quickly
4) riding with people who are fast makes the time go quickly
5) ice cream is awesome, ice cream and flapjack doubly so.
I´m planning to go a bit easier on my next 300, and try not to get caught up with trying to keep up with quick people at the front. There were a couple of points on my last 200 where I was well out of my depth, and I want to feel like I´ve done more than my fair share on the front. Also planning to do some of it on my own. Hopefully it will prepare me if I have some time between groups on the 600.