one thing a great many old bike people need to do is raise your seat so your knees are just slightly bent when the pedal is at the bottom of the stroke. I have a buddy who has no car, and only rides a bike. he thinks you adjust the seat so when you stop at a stop sign your feet are on the ground. he rides all over town at 3-5 MPH using all the wrong muscles.
hydrate or die. even riding slow on an old crusty bike you need water before during and after a ride. got a valuable lesson on hydration on a 10 mile Mountain bike ride several years back. 2 bottles were not enough due to the extreme hills I was not expecting on a new trail. plus it was hot. bought one of those bladder water backpack thingies before the next ride. now I have an even larger one that I either freeze or fill with ice before I go.
my buddy with no car NEVER drinks water while riding. offered him a big glass of water last time he rode by in the heat, he took only a few sips. dumb&#!^. gonna see him on the side of the road one day passed out from heat stroke.