This was driving me nuts…
It is there, but you have to be on the activity page, i.e. citystridesDOTcom/activities/1234567
On that page you can right click your profile picture, top right, and delete or edit the activity.
I have deleted some activities (with corrupt GPS data) and I get the note that those activities are schedulded for deleting … how long does the deletion take? Do I have to do something else (like staring a sync manually) to start the deletion process?
AFAIK, you don’t have to do anything else.
Maybe by now they are gone?