Eliminate old activity

Strava doesn't offer a way to "batch update" a large number of activities... they probably figure it'd cause more problems than it's worth.  But there is one third-party vendor which does: Activityfix.  He has a dot com website you can log into while logged into Strava.  You can set up a rule where you select all runs between a date range, change the activity type to something different (like "workout") and that will function in the background, maybe overnight.  That should wipe out your personal bests, course records, and KOM's, and career distance/time.  

I almost hesitate to recommend that because it violates one principal of data processing in that you are unable to make a backup copy of the data prior to the update.

If you don't want to do the above, have you considered creating a new Strava account?  That would restart all your personal records.  They don't want users doing that but people do it all the time for whatever reasons.

       
I know this was wordy but you asked a fairly complicated question.

In addition to what ​@JBW-Florida suggested… there's an existing conversation on this topic which you might want to join: 

And, there's also a feature request that could address what you need - consider voting for it: