Two of the issues for staying alert and awake are rehydration and refuelling. The brain is affected by dehydration as much as the rest of the body. The brain also is a heavy user of energy.
So, work out how you are going to keep enough fluids and food going into your system. Unfortunatey, you don't seem to have enough time to experiment with eating while riding, so you need to be aware that your stomach might not be happy with solids, and vice versa it might not be happy with liquid sustenance, either.
To assist with whatever food intake you choose or feel comfortable with, ensure you keep up your electrolyte intake with your fluids. This plays a dual role -- replacing salts lost from sweating, but also importantly, ensuring the electrolyte balance in your stomach is such that you can continue to absorb nutrients from your food intake.
Pace yourself. Don't go out earlly at a high intensity or pace. Judge your conditions, and settle on a pace that feels comfortable -- you should be able to have a conversation with someone riding alongside you without labouring your breathing.
Riding with someone also is useful when periods of tiredness start because it means having to think about conversation. If you don't have anyone to ride with, think about something -- calclate your average speed so far, what's left, etc, or play some simple mind games.
And if you do need to nap, try to do so in 10 or 15-minute increments, rather than 30 minutes. If you need to sleep longer, the best period seems to be 90 minutes.
That's all I have for now.