Perhaps you're rides are at too high an intensity. It may be a short full intensity ride is more exhausting then much longer medium intensity rides. With the right gears most reasonably fit middle age and youngish seniors should be able to handle longer rides then yours with all but the steepest hills with no more then medium-high effort. The nice thing about hills is the coast down. Perhaps you need lower gears. My lowest gear brings me down to about 6 kph @ 60 rpm.
Also it may take a couple months to get used to a an increase in activity. It might be a good idea to get a complete physical too, Tell the doctor what you have told us. It may be you're exhausted more from work the the bike ride.
Last edited by xroadcharlie; 04-21-22 at 12:27 PM.