Subway and diners are the restaurants I eat ate most when touring. I like Subway because you can load up on the veggies and it's a pretty healthy fast food option and you get some carbs for the pedaling. I'll usually eat half of a foot long sandwich and save the rest for dinner.
I like diners for breakfast because of all the people you can meet in them, they are local and the antithesis of the chain restaurant......the food is very variable, but you often come on some well cooked stuff at very reasonable prices and a bottomless coffee cup is great.
certainly agree on Subway, eating a good balanced diet of veggies and fruit and everything else is even more important doing extended physical exercise day after day, and a Subway sub is certainly better than only eating hamburgers or other fast food stuff.
also very much agree about what you say about diners, going into small places are always so much more interesting than a chain restaurant and makes going through small towns fun.