Eat whatever your favorite healthy diet is when off the bike. Just be aware that within 3 to 4 hours of a long hard ride, filling your gut with certain stuff might detract from your performance. During the ride, you probably need some carbohydrate supplementation. After the ride, its' said that your body is able to absorb/process stuff at a faster rate for about 45 minutes to an hour, so make sure you get some good carbs and proteins during that period.
I typically use a maltodextrin product to put in my bottles for carbs while riding and just after. It doesn't get used for any other part of my normal diet. So why should I care if it's "healthiest"..... I can't stand threads that use "best" or any "....iest" term. Have I ever said that before or can anyone tell? <grin> The rest and majority of the time, I eat a lot of vegetables, fruits and some meats.