If you completed a planned session another way (for example on your treadmill or with your watch) and it didn't sync automatically, you can mark it done without running it live in the app.
Open the session from your Plan.
Choose to log it as completed.
Enter the details. For tempo and interval sessions you can add each block. Other runs use a simple distance/time entry.
The session is then ticked off in your plan and added to your history.