This can be easily achieved by setting maximum run durations for the days of weeks concerned within your Training Settings:
1. Go to your Training Settings;
2. Click the ‘Update training settings’ button.
3. Scroll down to the day that you have limited time and enter the maximum number of minutes you have available to run within the ‘Max minutes for run’ field. (You may need to click the ‘Custom’ radio option in order to get the field to be displayed.)
4. Scroll to the bottom of the screen and click ‘Save’. Your plan will then be rebuilt adhering to your changed constraints.