If you are serious about your training, you may want to invest in a sport watch. The built-in heart rate monitor helps you keep track of your heart rate.
From Amazon
Garmin fēnix 6S PRO, Smaller Multisport GPS Smartwatch, Advanced Health and Training Features, Ultratough Design Features, Up to 9 days battery life, Black
Sigma Sports
Garmin Fenix 6S Pro GPS Watch
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