If you've been using a gps watch for any length of time, then records of your workouts live....somewhere. What you used 3 years ago might not be what you use now, especially if you changed from Garmin to an Apple Watch.
When I say "it takes 42 days to establish a baseline for you in Training Peaks," this is generally how long it takes for the system to get to know you and produce reliable assumptions about your fitness level so I can set appropriate paces for hard workouts and determine whether or not I'm overtraining you. I know this is frustrating for you, especially when you get emails about changes to your threshold, revealing numbers that don't look anywhere near accurate.
An easy way to fix this is with an app called RunGap. If you buy the upgrade for $3, you can connect all of your fitness accounts to RunGap and then push workouts back and forth.
For example, I used to used MapMyRun to plan my runs from 2007-2010, then received my first Garmin (2010-2012) then migrated to a Polar (2013), then migrated to a GPS Polar (2014-present). I got my own Training Peaks athlete account in 2016, and wanted my dashboard to more accurately reflect my running history. RunGap pulled all of my old workouts from MapMyRun, Garmin Connect, Polar Coach, and Polar Flow into Training Peaks and VOILA! Better data!
I'm not saying you have to run out and buy this app. Just letting you know that if you have been logging anywhere, it is possible to migrate your workout history into Training Peaks so I can get to know you better, faster.