Carb cycling and running

Good question, and i'll answer based purely on a nutritional basis, not from a 'splurge' day. To me that sounds like throwing the diet out the window for the day :P

So if you're doing either activity, to a fairly strenuous level (say zone 3 on a HR monitor / 75% max HR ish) you'll be expending approximately 2-3 times the amount of energy running than you will cycling.

The 'rule of thumb' for cycling would be to eat 30-60g of carbs per hour. Be it fruit/gels/sweets etc. Combined with this, you'll want to be drinking around a 1L of water/electrolytes.

Fats contain approx 4 times the amount of calories gram for gram over carbs. So consider this for your longer endurance events (peanut butter, for example)

So the bottom line is, if you havent drunk within the first our of exercise, prepare to bonk by the end of the second hour.
If you havent eaten within 1.5 hours, prepare to bonk by the end of the third hour.

The last little bit i'll say is that eating stuff that is carb heavy isn't the kindest on your guts in the beginning. Over doing it will no doubt end up in explosive episodes on the toilet, vomiting, or both. Work up slowly.

HTH!

Maybe the day before your long run day so you have the energy for it? Either that or the long run day itself...not very helpful but I'm just thinking of my own experience.