How to determine how hard you should do cardio exercise

I am a believer in using a heart rate monitor for cardio exercise. I started using one more than 10 years ago when I went on my mid-age fitness craze (I had just turned 40). In fact, I got down to 149 pounds! I was skinny as a rail and doing hours of cardio each week.

As I've mentioned in my chronicles, I am tracking how much time I spend in the 105-135 BPM zone. Over the years, I've found that when I train hard, I need to stay in the 140-155 range. Anything above that, and I just blow up.

This is very evident during the hot months. I can start a run and try to run a steady pace. What happens is that the HR will start creeping upwards very rapidly when the temps and humidity are high. And, in order to keep the HR in the training zone, I actually have to slow way down.

However, I consider it a critical piece of safety equipment, especially the older I get. Heck, I KNOW people my age that are having heat strokes, heart attacks, etc.

I digress........ Based upon the HR calculator, this is where I am supposed to be training:
(I am 52 and have a resting heart rate of 68)

Using the Karvonen formula:
Your maximum heart rate is: 168
Your maximum training heart rate is: 148
Your minimum training rate is: 128

They are telling me to train 128-148

I will eventually get there. For right now, I am going to keep with my training plan and keep increasing speed and incline until I work myself into their zone.