My wife and I are, lets just say getting older, 58 and 60.
My wife occasionally goes to the gym but not enough to get much benefit. She also was doing some Zumba which i think is great for her but stopped because she didn't like the teacher.
We live a long a way from any snow (San Diego) but used to do some skiing both DH and XC.
Also going to the gym is problematic especially in winter because there are a lot of sick people there.
I used to do some running, actually a lot and would run 2 - 3 days a week but can't do that and cycling now.
I think the best way to stay in shape for is cycling is, you probably guessed it, cycling. Especially if you have limited time like we do.
I don't want to buy two trainers and since my wife is so used to riding the tandem I think it would be more motivating for her if we were riding it on the trainer vs singles.
I am the one who plans all our rides, she follows along and isn't motivated to go riding on her own.
I don't have too much of a problem riding after work even in the dark and cold with the proper clothes and lighting, but my wife really finds it difficult.
I realize it isn't as much fun as riding on a sunny day.