Wrong basis
RyanM said:
I am 28 so my max HR calculated using one of the many formulas is 193.1bpm
Zone 3 (70-80% max) is 135.17-154.48bpm
Zone 4 (80-90%) is 154.48-173.79
Zone 5 (90-100%) is 173.79-193.1
What zone should I stay in for majority of my ride to increase endurance and get fitter? Zone 4 or Zone 3?
First of all, the formulas for max HR are essentially useless for what you want. Instead, base your training on your maximum sustainable HR (essentially your Lactate Threshold or LT) - what you can average in the last 20 minutes of a 30 minute all-out effort (time trial). From there, here are some zones you can calculate:
Active Recovery - Zone 1 <80% LT
Endurance - Zone 2 80-89% LT
Tempo - Zone 3 90-93% LT
Subthreshold - Zone 4 94-99% LT
Suprathreshold - Zone 5a 100-102% LT
Aerobic Capacity - Zone 5b 103-105% LT
Anaerobic Capacity - Zone 5c >105% LT
Using this procedure, zones 2 and 3 are what you want, but you need to go into the higher intensity as well (one or two days per week, do intervals) to build speed.
BTW, I know you did those numbers on a spreadsheet, but there is no such thing as a 5 significant figure heart rate. Some would argue that when you are exercising vigorously, there's maybe 2.5 significant figures.