Tips for long journey (40+ hours)

I will be traveling from North to South next month, and I don't know much about long-distance train journeys. So far, I have only travelled by train for a maximum of 5–6 hours. Please help me with the following:

1. Is there any way to book a ticket 2–3 days before the journey? I don't have an exact travel date yet, and the trains are already showing RAC.
2. I will be traveling in 3AC. What essential things should I carry with me?
3. How can I get food onboard the train?
4. Are the washrooms clean enough to use?
5. Will my luggage be safe in 3AC, or are thefts common?

If there are any other important things I should know before such a long journey, please let me know.