how to combine two activities

There is no way to do this within Garmin Connect. However, this still can be accomplished...

Did both activities upload successfully into Garmin Connect Web?

• If yes, then open the first activity, select the gear icon (top/right of page), then choose "Export File."
• Open the 2nd activity and do the same.
• Both activities will now be saved to your downloads folder as zip files, unzip them and save them to your computer.
• Go to FIT File Tools and launch the "File Combiner."
• Drop or select your fit files into the "Step 1" section.

• Click GO, then the link "Click here to download your file."
• Import the new "fitfiletools.fit file into the Garmin Connect Web import page - https://connect.garmin.com/modern/import-data

Note that Garmin Connect might reject it giving you the message "This file has already been uploaded." This is because the new activity has a start time that is identical to the activity that was the start of the marathon, so you'll need to delete that one then upload the new activity again.

FIT File Tools doesn't always work, in which case attach the zipped files to this thread. We can take a look to see if it is repairable. 

• Zip the file, then in a reply post Insert >> Image/video/file >> click Upload >> browse for file >> OK.
• Or upload the fit file to a file sharing service, such as OneDrive, Google Drive, Dropbox, make the file public, then paste a link to the file in your post.