Originally Posted by Ric Stern
... when you are untrained it's comparitely easy to work at an intensity that elicits a high(er) rate, however, as you start to gain fitness it can become psychologically harder ride at an intensity that elicits that same HR as you now have to push considerably harder...
I'm getting to that point now. It seems that I'm having much difficulty sustaining 85% MaxHR for an hour, whereas I used to be able to sustain 85% MaxHR for 2 to 3 hours! Keeping my HR at that level is much more exhausting now.