Tap Send to Watch, then pick your watch's companion app from the share sheet that pops up. The route imports as a "course" or "route" and syncs to your watch the next time it connects to your phone over Bluetooth — usually within a minute. After that you can leave the phone at home and let the watch guide you turn-by-turn.
Apple's built-in Workout app cannot import GPX routes, so you need a third-party app on the iPhone with an Apple Watch companion. From best UX to most fiddly:
Cheap workaround with no extra app: tap Download GPX, share it to Files, open it from Files, and Apple Maps will show the route as a static overlay you can glance at. Then start a generic Outdoor Run from the watch's built-in Workout app. Not turn-by-turn, but uses only Apple's apps and costs nothing.
Tap Download GPX instead, transfer the file to your computer, and import it through your brand's web dashboard. Almost every fitness watch supports GPX import somewhere.
Draw your running route directly on the map. Each tap snaps to the nearest street, and consecutive points are connected along real roads. Once the route is set, you can drag any point to make it longer or shorter — no need to start over.
Tap the map where you want to begin running. The first point becomes the green Start marker and is saved on your device for next time.
Every additional tap adds a purple point and connects it to the previous one along real streets.
Once you have at least two points, tap the Start pill to turn it into Start/Finish. The route is closed and brought back to the starting point.
Grab any point — including Start — and drop it elsewhere. Only the affected segments re-route; the rest of the track stays put. This is the easy way to make the run longer or shorter.
Tap Undo to remove the last point. Clear wipes the whole track but keeps Start. In fullscreen mode the Undo button sits in the bottom-right corner of the map.
When the route looks right, tap Send to Watch to push the GPX into your watch app (Garmin Connect, Coros, Komoot, etc.). Your watch syncs from there and you can leave the phone at home.