Doc wasn't intending to go to November 5, 1955 with his first trip. His desire was actually to go to the future:
Marty: The future, it's where you're going?
Doc: That's right, twenty five years into the future. I've always
dreamed on seeing the future, looking beyond my years, seeing the progress of mankind. I'll also be able to see who wins the next twenty-five world series.
In the movie, Doc was demonstrating to Marty how the time machine worked. He showed Marty how to program the time machine and used three example dates of important moments in history, showing Marty how to enter them into the machine. The November 5, 1955 date was simply the last of the three examples that was entered.
After Doc got into the machine for his first trip, he could have entered any date he desired before leaving. Unfortunately, as we all know, his plans were interrupted by the Libyans, and Marty accidentally traveled to the last date that was entered into the machine.