Allow range for workout targets
When creating workout targets in the workout builder, allow setting target values for duration in range.
For example, currently instead of 10 min warm up at 60%,
allow setting 10 min warm up at 60% - 65% (this is usually how coaches specify target values..).
This should also apply for actual values, not just percentage of TH. e.g., 10 min warm up at 100 - 120 bpm / 120-150 watt).

March 14, 2018
The workout builder has been updated with three new features:
1. Workout steps can be set to have an open duration. For most devices pressing the lap button will take you to the next step of the workout. This is currently available for Garmin devices. We expect other device manufacturers to implement this feature shortly. A suggested duration is still required for these steps so the planned duration and TSS for the workout can be calculated. This value will also be used for devices and file formats that do not support this feature.
2. Target values for the step (power, pace, heart-rate) can now be set to a custom range. This range will be used for Garmin devices. Most indoor cycling platforms require a single value for ERG mode, and the midpoint of the range will be used in that instance.
3. Cadence can now be set as a second target for each step of the workout. Currently cadence is supported for bike workouts in Zwift. We expect other device manufacturers to implement this feature shortly.
Not every device or platform will support all of these features. Here is a short list of partners and which features each one supports at this time.
https://help.trainingpeaks.com/hc/en-us/articles/360001652912
The following FAQ pages have additional information on these new features.
https://help.trainingpeaks.com/hc/en-us/articles/235164967
https://help.trainingpeaks.com/hc/en-us/articles/115003385172
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Christian Dela Cruz commented
After launching this feature, the auto calculation of total distance does not work anymore even though duration and average pace are auto computed. Please fix.