Squat Rx #27 - 20 Minutes of Squatting

As I age, the things I enjoy and am proficient enough at to do for an extended aerobic period of time are limited. I respect running and I think running would be great, but it's not a thing I do. I just never developed a taste for it. Swimming would work, but these days I'm about 25 pounds too heavy to find getting into the pool even close to enjoyable. Walking the dog is a possibility - I enjoy it, but if I allow the dog to sniff, pee, and poo at her leisure, there's no chance of raising my heart rate enough to approach "conditioning". I still do kettlebell snatches, but infrequently and I've lost a step over the past 5 years or so of little to no practice.

So, I'm left with squatting. I'm proficient and practiced at it. I enjoy it. It doesn't hurt me.
The other day, I squatted for 20 minutes with 135 pounds. The goal was not to get as many reps as possible in the time allotted. The goal was to keep my heart rate under control for the duration. It was harder than expected and an exercise in discomfort and patience, but I managed to keep my heart rate in the 70-80% range (with a few blips above) for the entire 20 minutes.

Sometimes I'll just squat without a timer or heart rate monitor and just squat for a certain number of reps, or I'll use only a clock, but for this, I used the following tools:

Polar Heart Rate Monitor: https://www.polar.com/us-en/products/accessories/h10_heart_rate_sensor
Online 'Analog' Pace Clock: http://www.online-stopwatch.com/pace-clock/
Gymboss Interval Timer: https://www.gymboss.com