Hi
It is not necessary to turn heart rate on or off on a per workout basis:
On your iPhone, in the Watch app, go to: My Watch > Privacy > Motion & Fitness - check that both Fitness Tracking and Heart Rate are turned on.
Selecting your activity type in the Workout app enables Apple Watch to use the most appropriate sensors and data sources when estimating the results of your activity.
For all activity types, the Workout app automatically monitors your heart rate continuously during your workout. This information is used when estimating the results of your activity.
To credit progress towards your daily Exercise goal, the Activity app aims to identify activity that equals or exceeds the intensity of a brisk walk. This requirement applies both during general daily wear and when recording workouts via the Workout app. Exercise credit is awarded for each full minute during which this requirement is met.
The definition of a brisk walk varies from person to person and depends on your personal information.
To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
Calibrating your watch can improve the accuracy of estimations made by Activity app and Workout app. More information and instructions for calibrating your watch - which can only be done during Outdoor Walk and Outdoor Run workouts - are available via the link below.
If you would prefer to receive Exercise credit for the full duration of your workouts, regardless of their estimated intensity levels, track them via the Workout app using "Other" as the activity type. This will credit one minute of Exercise for each full minute of the workout. Active calories will be estimated at a rate equivalent to a brisk walk or based on data recorded by the heart rate sensor, whichever is higher.
More information:
Work out with your Apple Watch - Apple Support
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Use the Activity app on your Apple Watch - Apple Support
