I believe that if you change your privacy settings so that the default is “only you”, it will only apply to new uploads and then you can individually select which new uploads are public.  I am almost certain that it won’t go back and change all of your past uploads, but someone else might know more definitively on that.  

I appreciate your response. I will give it a shot and see what happens. I suppose I can always reverse it if it retro hides my previous activities. Cheers!

That's correct. It's two different settings - the default privacy option is only for new activities. For already uploaded activities, there's a separate mass update feature.

I record my activities using Strava on iPhone and on Apple Watch, and I finally deciding to record with default privacy setting to “Only Me” so that I can later check which track came out cleaner before making one or the other visible to “Followers” and deleting the other.

I have a couple followers who apparently live 24/7 on Strava, because if I make an activity visible just before entering my building door, there’s a good chance I’ll receive a kudo by the time I finish the elevator ride upstairs to my apt. (I am not joking. It’s happened. Multiple times.) So if you need to pick and choice your activity settings, best to set the default to private, and then edit as need be.