'basically walking' still allows for gains in fitness and cardiovascular health. It is well worth slowing down to a walk or even a walk with breaks if you are concerned.
FWIW, I remember coming back from health problems and not being able to make it up and down the stairs without having an elevated heart rate and it was very frustrating. I started with walking, and it did allow me to go up and downstairs with much more ease. There is no way I could have hoped to be where I was fitness wise 6 months earlier. I then walked farther, then I did it while pushing a stroller, then I decided to go farther with the stroller, then I decided to ditch the stroller and run for 1 minute of a walk, etc. I have come a long way from where I was, and I am still not where I think I could be. Maybe it is fine to walk for a bit, or switch to a bike if it is easier to modulate your effort to control your heart rate?