Last week during the burn challenge I worked out on the stair mill, it get's my heart rate up 5 minutes or less. I kept steady at a level 3 for at least an hour or more each day.
I noticed last Saturday I could only get my heart rate no higher than 123 when at the start of the week it climbed and held steady at 133-141.
This tells me my heart's getting stronger.
On Monday I decided to up it a level and see what happens. At level 4 my range was 119-131, still not good enough so Tuesday I decided to program the stair mill at a level 5, I managed to maintain that pace for 25 minutes, then a level 4 for 20 minutes and then a level 3 for the last 15 minutes. My heart rate ranged 131-144, even though I didn't maintain a level 5 the entire time.