Yams, sweet potatoes for the carbs and nuts, nut butters, oils. Neal touched on this but there are certainly calorically dense foods that make getting that 5k doable. A can of coconut milk has ~800cals alone. You may need to up your carb intake to ratchet up your appetite. Paleo is just a tool to use if you want to increase phytonutrient intake and minimize allergen/autoimmune potentials. I think it is pretty easy to take in adequate calories on this plan, but if it seems too hard go a more conventional route. That said, rice pasta and the like are not more calorically dense than the aforementioned foods, but they are a fraction the nutrient density.
"Survival will be neither to the strongest of the species, nor to the most intelligent, but to those most adaptable to change."