What "late" said.
What you need to do is go on some rides that are long and fast by yourself and go on other rides that are short and easy. I notice around here, for example, that on some of these rides, they'll have even a 10 mile "Family Ride" or something of the sort. And you'll find that many of the ride organizers seem to seek out hills to make it challenging, but some don't, so the courses vary that way too. But intentionally make efforts to arrange trips that are conducive to her abilities.
The tandem is an idea, but before considering that, you'd probably want to talk to folks in the Tandem Forum. If one rider is out of shape and one isn't, I don't know that the end result would be a whole lot different emotionally. You want to ride, not go dragging another 100+ lbs of dead weight around with you. If she was willing to get out and ride more locally on a tandem, then it might be worthwhile.
Even if your wife makes an effort to get in shape, there's likely to always be a considerable disparity in how far and how fast you can ride and how much you enjoy doing so. So her getting in better shape is helpful, but not an entire cure to the problem.
__________________
"be careful this rando stuff is addictive and dan's the 'pusher'."