Foamy,
Average speed questions are much maligned, but I can give you a single point of reference; my wife and I are senior citizens (129 y/o) and have been riding for a bit more than a year. We've had our tandem since last May. We averaged just under 18 mph for a flat 100K. YMMV and hills really slow us down. 8-10% grade and we're not good for more than 4 mph on a sustained climb. Our lowest gearing is 24-34.
We enjoy the tandem tremendously, but I'm in a bit of a slump. Sustained climbing takes a lot of mental fortitude and I can't be thinking about how much faster I would be climbing on my single. It's a team thing and the focus needs to be on the sense of accomplishment in getting to the top and the fun we'll have going downhill. My one regret is that I can't turn back the clock about 25 years when we would have been really fit!
Sounds like you're off to a good start!