If the in-laws have to rest or slow down simple suggest/ask (have hubby ask) would you like to relax at the hotel for awhile while we hit one or two more things... or do you want to stay at the restaurant or cafe longer while we quick see 'whatever site you want to see in the area.'
Also plan out your 10 days, give time to travel to each locations, as well as eating. Ask the in-laws how much they would like to see in a day (maybe 2 big things a day) and what they REALLY want to see. Then starting planing the days with everything they want to make sure they see. Then ask (hubby ask again) if they would be ok with you guys see more sights (or hiking) without them. Morethenlikely they will realize they can only do so much and that you two are younger and can do more. As long as you for sure see what they want, in a timely planned fashion and then you and hubby break away while the take a break (nap, eating longer lunch, tea/coffee break, they walk through a market, whatever) most would see that as reasonable. Also if you want an hour or two by yourself, have hubby take them to a place that you are fine with missing out on (yet another castle, or another flea market). That will give you the needed break... when we did family vacations we all got along time at some point to just nap, have coffee, sun outside whatever we enjoyed and that really helps keep everyone sane on a longer vacation.
If this is a for sure thing I would make the most of it... maybe find a romantic place or two that JUST you and hubby can go to (dinner whatever you find romantic). Find a few things that you REALLY would like to see, seriously ANYONE can find at least 2-4 things in another country that they would LOVE to see. It seems that your in-laws are saying you guys are in charge and just thankful they do not have to do the whole trip by themselves.