What Causes Elevated Heart Rate In An Elderly Person?

I m 65 and somewhat active. My AM heart rate - taken in bed before rising - is 65bpm or less, But simply sitting, it rises to about 90 later in the day, with a walking rate slightly 100-110bpm. I m slightly overweight, but otherwise healthy, no prescriptions, but do have some asthma. The numbers given are without the effects on any asthma drugs though. Is this a reasonable range? are there any guidlines for active (ie. walking) heart rates for age?