My wife and I (60++) did a century a few weeks ago on the tandem. Not too tough a ride with 3500 feet of climbing mostly along the California coast, but with the last rest stop at about 80 miles my wife was toast and the remaining ride saw temps 95-100. There were two gentle climbs that ordinarily would have been nothing, but we were crawling. Went for a 20 mile ride 3 days later and every muscle in my legs hurt, not as bad as post-marathon, but as bad as I've ever felt on a bike. I drank like a fiend on the century, took electrolytes and didn't cramp so I have no idea what brought on all the muscle damage - must be just old age, but I'm open to other opinions.
There is nothing so psychologically daunting as dealing with real pain on a tandem, unlike a single where you have no one to blame but yourself.
The good news: we did 47 miles yesterday and I felt pretty decent.