Insufficient preparation

In July, I'm planning on riding my bike around a hundred miles a day for four days. I've done virtually no riding, other than my short commute, this year so this is possibly a Bad Idea. But, I quite like bad ideas.

I've got a few rides in over the Easter break so far: I borrowed my brother-in-law's bike while we were staying with them, and have managed three consecutive days of shortish local cross rides since we got back. I've been reasonably quick on these rides, and don't feel too beaten up despite riding rooty singletrack and rough tracks on an 'inappropriate' bike.