Your history
Import your runs
Upload the data export Strava or Garmin emailed you, as the zip it came in. Your past runs give the coach your training, paces and PRs from day one. Nothing is analyzed until you open a run.
Getting your export
Strava
On strava.com, go to Settings → My Account → Download or Delete Your Account, and choose Request Your Archive. Strava emails a link, usually within a few hours.
Garmin
At account.garmin.com, open Data Management → Export Your Data. Garmin emails a link, sometimes after a day or two.
Archives over 1 GB (usually from years of Strava photos) won't upload. The newest 1,500 runs are read; runs already in Pacehound are skipped.