That would be my advice but if you want to do it- then go ahead. Just remember that you do sit down a lot more on a Tandem and you have to communicate to get Butt rests. A Stoker suddenly moving to get comfortable can put you in a ditch very easily. On the saddles-Take your own and they may be better than the Hire Tandem ones but I have found that a Tandem needs a wider more cushioned saddle than on my solos. SCH has it right in that Saddle soreness will arrive. Now about those hills- Enjoy them if you can. You will have plenty of time to look at the scenery- providing you can raise your head off the bars with the effort they can take.

Just let us know if you are still talking to your sister after the event as 100 miles as a first ride might be that step too far.