I can still remember 11 years ago when i decided to go shopping on my normal bike for the first time in 14 years. I glided all the way down the hill to the town centre, thinking how brilliant bikes are as a means of transport. I filled up my hold-all with shopping, then started the 1/4 mile continuous climb back up the hill. I got about half-way up before I couldn't breath. I literally got off and lay on the ground gasping. Luckily, there was no one around, otherwise they'd have called an ambulance.
8 years later, I was doing 60 mile rides that included some of the steepest climbs on my road bike and clocking up 1000 miles a year on my electric bike..
Today, I can't even get on my bike without gasping for breath, but that's another story and hopefully going to be temporary.