The biggest problem I have on long rides (and for me a 200k is a long ride) is remembering to eat and drink enough.
Daft as this sounds to anyone who has not been there, this is a very real and common problem. I have done nearly 200 audax events, and I still sometimes get hydration wrong. The common comment is "by the time you feel thirsty it's too late".
Good luck 28spoke. As the Marmite man says, it's addictive.
For me when I'm making good progress I don't feel the need to slow/stop to take on food and water, but then suddenly find I'm not making good progress and find myself checking my trip meter more and more frequently. When I realise that I'm checking my trip meter more than once per kilometre it's a good sign that I need to take on food and/or water (usually both).
At present I can ride at a high sustained effort (high for me anyway) for a good couple of hours without taking on any water, but then I'll drink like a camel at the end of it. That works up to a point, but if at the end of my two hours I'm expected to continue to ride for another two hours, even at a lower pace, it starts to become a problem because by then I need to give my body time to process the water.
Eating little and often is much easier if you've got a way to eat while on the move. I usually carry bags of jelly babies from Lidl (59p for 250g) which are the best way I've found so far to get lots of calories for little money while still tasting nice (and to me they taste nice rather than just passable). If, as I've often done, you leave your food in a pannier or saddle bag and have to stop to get at anything it seems like a faff and you're less likely to actually do it, leaving you more likely to bonk later on.
If you can't already do it, it's worth learning to drink while on the move, even if you can only do it while freewheeling it's much faster than having to stop, drink, get moving again.
