Please manually enter the duration you did for your cross-training and leave the distance field blank in the workout card. This will allow the activity to be marked as complete as well as the duration to be taken into account for your training volume.
To manually enter the completed duration:
1) Tap on the workout, this will open the expanded so you can see more details.
2) Tap the pink "+" button which will open a few options.
3) Select "Manually Log Run"