Machka, your past long distance experience is both a blessing and a curse. Folks taking up cycling (or any aerobic activity) for the first time don't have expectations and seem to set modest goals. They get tired, they stop. They feel pain, they stop. This is probably not a bad recipe for restarting after a long layoff, but those with an athletic background and high expectations are used to pushing themselves. I haven't run a marathon since age 40 (24 years ago), but the mentality to push until you drop still lingers after more than two decades of declining athletic activity. Fortunately, my schedule and that of my wife provides some limits (we ride a tandem). We will never at our age be able to climb with singles, but we can ride 100K and 100mi. with a reasonable amount of climb, and we can ride it together.

You ought to give a tandem a try. By all indications you and Rowan have a wonderful relationship and if you're comfortable in the stoker position (on a "normal" tandem) you will each be able to work in whatever zone suits you on a given day. Swing by the tandem forum and you'll get plenty of advice, mostly all good. Lots of psychology to sharing a tandem, but when it works it's a great experience.