If you are serious about your training, you may want to invest in a sport watch. Use the built-in heart rate monitor to keep track of your training progress.
From Amazon
Garmin Vívoactive 4, GPS Smartwatch, Features Music, Body Energy Monitoring, Animated Workouts, Pulse Ox Sensors and More, Black/Slate
Watch Shop
Garmin Vivoactive 4 Shadow Grey with Silver Hardware - LCD / Grey
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