Good luck at the race!
Regarding your Paleo diet, it will take your metabolism a while to become efficient at fat oxidation. If you are not there yet you can still stick with Paleo and do medium-high carbs, just get them from yams, sweet potatoes, fruits, squash etc.
For your next training session you could experiment by adding some easily metabolized fats like medium chain triglycerides (MCT) with your usual food. E.g. make a shake with some coconut water (which also contains electrolytes) and some protein in addition to your glycogen replenishing carbs.