I capture the Strava on my smartphone and have no idea how to edit the tracks. I don't mind having more than one track per day but just thought that it would be nice to bring one ride together if it was easy to achieve.
First google hit for 'join gpx files online':
joewein.net/bike/gpxmerge/
Download each activity from Strava, save each to a folder on your computer, then go to the above url, locate the files to join. Once you've selected the input files, click to export the joined gpx, save back to your drive. Upload to Strava, and check that the resulting activity looks correct. Assuming it does, I'd be happy to delete the original activities off strava.
I haven't tested this method myself - there are extensions in the file I downloaded for my own ride this morning for heart rate - I have no idea if that will be handled correctly.