I run a little bit but not 15 miles/week. More like 6-9. The best thing I’ve found is eating sugar (like OJ, dried fruit, honey) before exercising. And getting adequate sleep. In fact, if you’re hoping to gain exercise ability and not pack on pounds, building on what David said, you may want to aim for more high carbs and lower fat. If you were low-carb, starch may be more tolerable than sugar at first, but realize that sugar gets temps up fast for a lot of people, and much more than fat.
It sounds like what you’re eating right now is pretty high fat, which is why I point this out. If it were me, I would be eating more like a lean-ish burger (no mayo) on a bun, baked potato or sweet potato with a little butter or yogurt on top (instead of the fries), and OJ or other juice to drink, and maybe some sweet fruit like pineapple or mango, dried fruit or a little ice cream or sorbet for dessert. If you start feeling like you’re not getting enough fat, you can always add more, but you’re taking in an awful lot of fat (and especially PUFAs) right now. Also, make sure you are getting adequate salt (this is not an issue with McD’s, obviously).
If you ruined your metabolism by eating low-carb, it may be more important for you to heal by getting carbs rather than by packing in huge amounts of calories.
And kudos to you for doing the tough mudder! I am way too much of a wimp, but I have friends who’ve done it and had a lot of fun.