OK, Steph - Get a hold of yourself/your thoughts. You'll be fine on this bike ride. Yes, you won't have ridden 70 miles prior. Yes, you haven't had as much training time as you'd like. But you've got all day and you're in good shape. Your aerobic base is strong and you're climbing really well. (Plus, you have lots of gears!) 74 miles will come and go. Just keep pedaling. You've got the longest days of the year to help you along.
This is not a race. You don't have to compete with anyone, not even yourself. The only thing you need to prove is that you can do this without being "perfectly" trained. You can have fun, eat an ice cream cone and enjoy the ride.
No hurry. No worry.