If you have enabled the link between JOIN and your Strava account, we will read at least (but more if available) 42 days of previous activities when calculating your training plan. This is a great help for us in compiling a high quality training plan for you.
Your historic Strava data will be synced immediately to JOIN as soon as you give the permission, but you won't see this history in your JOIN app, because we sync them in the background to help establish your initial fitness level.