Well, I finally broke down and went to a trainer. The only way my wife and I are going to be able to handle on our tandem the 6000' of climbing typical for most centuries around here is for me to get stronger. The trainer is an experienced cyclist and very familiar with the sorts of muscle imbalance issues that need to be dealt with. Trainer performed a pretty comprehensive evaluation: cardio, flexibility, strength, multiple caliper measurements, etc. The verdict: cardio was excellent, flexibility stunk and I need to drop some lbs and increase muscle density. Next step is to develop a weight program which will improve my overall muscle balance and, specifically, improve cycling strength.
I'm 65 and in a "use it or loose it" mode. At least I'll find out what I have left.