I had a similar no-so-great overnight trip last year in mid October. I figured few would be out camping, even though it was a pretty nice mid-October day.
Left around 11:00 Saturday for a 45 mile ride to a beautiful park.
I had phoned earlier and the ranger explained that sometimes the sites fill up. He was sure right. All camping sites were taken up as I arrived.
So I turned around and rode 45 more miles back home.
I did enjoy a late evening picnic, but had to do the last 20 miles on an unlit trail with a 1/2 watt headlight. Very interesting.
Lesson learned: head out on Sunday, not Saturday... the ranger told me that on Sundays, the park is usually empty.