When my career and life started slowing down enough in my early 50's I too started riding for fitness. Even just a ten mile ride was enough to have me coming home and soaking in a tub of cold water to cool off. However it didn't take too long to get use to cycling regularly and for long periods of time. I did my first 100 miler solo that same year... on a 46 lb 1979 Schwinn Varsity. I'd bought it about the time I had to stop riding regularly. And between 21 yo and 50 yo, I only used it for brief rides around the neighborhood.

The number one thing I think that helped me the most was figuring out that I had to maintain my hydration while cycling. I sweated a lot. And could loose as much as five pounds just on a ride of a couple hours in the heat of the summer. Realizing that, I began to weigh before and after a ride. And I adjusted my drinking habits to take a few gulps every 10 minutes. Eventually I started getting back from a ride with no more than pound of weight difference. Many times less. Most say they drink every 15 minutes, but I felt that I had to drink down too much if I waited that long.

Second thing that helped me the most was to realize that I had to ride at a level of effort I could maintain for the entire ride. Not maintain any particular speed.

Third thing was to realize that you shouldn't be pushing hard on the pedals when just cruising. It should be a ridiculously easy gear ratio to pedal. Back in 1969, the woman that owned the Schwinn shop and sold me one my first 5 speed derailleur equipped bikes had told me that pedaling should not be hard. Easy pedaling is better for the heart. And fast cadence with easy pedaling results in a faster speed than a slow hard cadence at 60 rpm or less.