Mel22,
Thanks for the post.
Concentrate less on your heart rate and more on the activity you perform. If you are able to go farther in the same amount of time, then you are "getting in better shape". I think you should congratulate yourself for being able to exercise for an hour! I would guess that is in the upper 95% of our readership.
Be careful attributing this finding to the ARB/diuretic combination. I have never heard of such a response, and a biologically plausible explanation is not apparent.
The simplest explanation is that you are better conditioned than you were previously.
Hope that helps.