Well, if you put down the hammer... what will happen if you crash and burn? You just walk the run?
Maybe it's time to test your limits? See what you can do? Might I even suggest that you ditch the HR monitor and just do what feels like a challenge but not completely out of your range?
If you go for it... what's the worst that will happen? And do you mind if the worst happens? Is the pay off of pushing yourself on the bike worth it?
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"