Here's a checklist of what you may need for the DK:
1. Four to eight large water bottles (preferably half uninsulated, half insulated)
2. Hydration pack: If you're a light sweater, you may be able to get by without this, but it's risky in my opinion.
3. A dozen snack-size Payday candy bars. I eat one of these an hour (alternating with gels).
4. A dozen gels (pick your favorite flavored goo and eat one an hour). Alternate, minimally tested choice: Ted's Untapped Maple syrup.
5. Skratch. I use it in my water bottles and keep my hydration pack filled with water.
6. Three gallons of water (for refilling said hydration pack and bidons)
7. six-pack of Coca-Cola. Long races are the only time I allow myself bug juice. Old-fashioned Coke, with its caffeine buzz and high sugar content, is hard to beat at checkpoints.
8. Pickles and bananas. The data on pickles for cramp prevention is light and inconsistent, but the salt tastes great on a hot day. The bananas are bland and smooth enough to still taste good at the end of the day.
9. Sunscreen. Don't even try to go without it. Reapply at checkpoints, too.
10. Butt lube. I like Chamois Butt'r. Have enough on hand to re-apply at every checkpoint.
11. Three extra inner tubes. This goes whether you're running tubeless or with tubes (I've done it both ways, and I can't say one is dramatically better than the other).
12. Patch kit.
13. Chain breaker. I hear stories of broken derailleurs every year, and if you can't shorten your chain your day is done. Get one that has an attached multi-tool or carry a multi-tool separately.
14. Extra chain link. Make sure you get one that matches the size of your chain, i.e., 10-, 11-, or 12-speeds.
15. Multi-tool. See above.
16. Extra brake cable if you have cable-pull brakes. This isn't to carry with you, but it's nice to have at a pit stop if things go wrong.
17. Extra shift cable. See above.
18. Chain lube. When you lube your butt, lube your chain. I like wax-based lubes because they're less finicky about wet or dry conditions.
19. Glasses cleaner and rags.
20. Extra water to clean your bike at checkpoints in case of heavy mud.
21. Mini-pump (or a frame pump if you're old school).
22. Three CO2 cartridges (one for every spare tube).
23. Tire levers.
24. Headlight, fully charged (so bring your charger).
25. Taillight, fully charged (so bring your charger).
27. Lawn chair.
28. Three moist towels folded inside plastic zip-loc bags. They'll be nice and warm when you take them out to wipe off your face between stages.
29. GPS, fully charged (so bring your charger). I know this sounds like a techie ad for unnecessary doo-dads. And yes, the race organizers do a good job with cue sheets. But having the course loaded onto your GPS is so much nicer. And later, you'll have the GPS data to use to brag to friends. And you'll be able to turn off your phone. Phone reception is non-existent for much of the course, anyway, and your phone will drain its battery trying to find a signal.