It was beginning to get dark.
He did not know how far they had to ride, but he hoped it was a long way.

Ordinarily, he would not have wanted the ride to be prolonged because his position was highly uncomfortable.

Fred could ride well himself, but riding alone on a horse and sitting behind a man who fills his own saddle with very little to spare are two different things.
Try as he would, Fred could not think of a means of getting away.

To escape from five mounted men by slipping off the horse and running for it was manifestly impossible.

He gave up that idea before he even elaborated upon it.