This is some of the reason behind Alpha: Challenges (request for testers) … I don’t have private Challenges, yet, and a starting date of 10 days ago isn’t something that’s going to resonate with anyone else (as opposed to a start date of Jan 1, for example) so you’ll have to hold off for private Challenges.
I didn’t build CityStrides as a “starting now” kinda project, I built it as more of a “lifelong” thing. That’s why it pulls in the full account history & deleting activities in CityStrides just get re-synchronized instead of staying deleted (caveat: full account syncing for Strava accounts isn’t automatically run, so that might not be obvious).