I think you emailed me about this as well.
My email response:
If you don’t care about the activity at all, yeah, you can delete it. When viewing an activity, the option is in the top right menu.
The Strava backlog is so huge though that resyncing it will take quite a long time. There’s no easy way around that, though.
Also, you should check this out: New: Garmin Integration -- Available To EVERYONE