Best way to determine THR is by using the Karvonen Formula. Simple to use and takes into consideration age and most importantly RHR (Resting Heart Rate).
1. First determine age related MAX HR to do so use the following equation:
220-age=MaxHR (ie 220-26=194bpm).
2. Next determine your average RHR (ie 60bpm) the lower the better. Normal range 50-100bpm.
3. Consider your approximate fit level. As part of the formula you work at a % of your max HR for example 60% for low fit levels 90% for well seasoned atheletes.
Now for the equation which is as follows: (MaxHR-RHR)[60% to 90%]+RHR=THR
Max HR=194
RHR=60
ie: 194-60=(134)[.85]+60=174bpm
The entire goal is not to get your HR up as high as you can, in fact the goal should be working on conditioning your body (heart, lungs, muscle) to sustain maximal efforts while maintaining the lowest possible HR.
Any other questions feel free to post them