Hmm. Wives and families do complicate things. Especially since, if you are working long hours they definitely want to consume the little free time that you have. And frankly, you probably want them to, too. But still, it's nice to get out now and again.
Ideally, if your wife is into 'adventures', at least you can both hike and spend time with her simultaneously. That's approximately ideal, Brother! My wife won't do more than a dayhike, and whines through almost all of it. :o)
I'm a physician, too. I'm quite familiar with 100-hour weeks. But I'm unusual, too, in that I'm an Army surgeon, so I get 30 days of leave a year. My wife and I have reached a certain detente, wherein I get to use one solid week of that to do a decent hike once a year. Then we tend to do a nice 2-week vaction, and we tend to break up the remaining week to visit family sometime, or whatnot. Also, working for the government I occasionally catch some long holiday weekends (if I'm not on call or have any patients admitted, that is). That tends to even out the otherwise long hours- and in a way that I find particularly beneficial to me.
It is EASY to find yourself working all the time. I certainly can't criticize a 6-day work week without being a monumental hippocrite. A few years ago, though, I decided that I wasn't happy and resolved to be a little more selfish. Primarily, I started guarding my scant free time very zealously. Other people will consume all of your time IF YOU LET THEM. They will abuse you, use you, and take you for granted. Believe me. Especially if you are salaried rather than working an hourly wage.
That may be an answer, actually. Figure out about how much you make an hour working these long hours, and ask your in-laws to pro-rate it to a 40-hour wage instead of a salary. You'll make a little less, but if they keep you late you can be brutal about clocking overtime beyond 40 hours, and potentially make MORE than you do now.
For instance, if you are currently salaried at, say $50,000 a year, that's about $962 a week. If you currently work 6 days for about 10 hours a day, that's 60 hours. 962/60= $16/hour. Ask to be paid hourly at that rate, instead of salaried. Granted, if you do only work 40 hours a week that drops your income to $640/week or $33,280/year. (But you'll have the free time that you crave.) And if they do keep you past 40 hours you make overtime. Then, if you find that you are still working 60 hours a week, at least you got a raise out of it. With time-and-a-half for overtime that 60-hour week gets you $1120/week or $58,240/year.
But as a first step I guess I'd recommend just going to your inlaws/bosses and tell them that you want to help them, but you aren't happy. See what they say. Of course, be politic about it- mention that you'd like to spend more time with their daughter, for instance.
The worst thing they'll say is "no." In which case they aren't very nice inlaws and you can say "I quit" with a more clear conscience.