I never eat a full meal when on the bike, no matter how long, unless I'm touring and it doesn't really matter when I get to wherever. That said, protein is important on any ride over 3 hours. However, there's no need for over about 15% protein. F.I., if you're going to ride say 16 hours and you eat 200 cal/hr, that's 3200 calories * .15 = 480 calories protein or 120g = total daily protein requirement just from on-bike food.

So yeah, eat a little protein or use a bike food that has integral protein, but mostly eat carbs and a little bit of fat. I do like "real food" like a Hostess fruit pie or a tuna sandwich or even a 6" sub if there's no climb coming up soon. It's good to put something in the gut for it to work on, but not too much, and never much at any one time, like a "full meal." Good way to be sitting on the curb or barfing in a ditch wondering what happened 10 miles later.

Chocolate milk is a good way to get some protein, as are sandwiches, etc. Ensure is really good. Main thing is to stay on the bike as much as possible and get up the road.

Chocolate is too much fat. "Sugary" isn't the right thing, either. I can't drink anything that tastes very sweet for hours and hours. That's why I use a lot of maltodextrin for carbs. No sweet taste, no taste at all really, which is good. I can eat a scone OK or a maple bar or Fig Newtons or a Snickers bar. Or granola bars or Clif bars or any number of things like this. You want edible calories, whatever goes down quickly without complaint in large quantity and isn't too fatty. Experiment.

Hammer puts soy protein in their endurance drinks, which bloats me up. I like whey better. Whey can usually be found in recovery drinks, so if I'm going to buy something, that's what I prefer. Recovery drinks usually have too much protein though, so I need to thin them out with some other carb or use them along with other carbs. I've used Hammer Gel for that along with malto and ordinary carb foods.