Only way to get accurate real time data when running is to use a good foot pod like Stryd. GPS just isn't as accurate as people think, no matter if you use Apple Watch or Garmin.
But if you have a clear line of sight that's a different issue. I went cycling on a path next to a river and the GPS trace was buttery smooth.
GPS.gov says single band receivers should be able to get less than 2m accuracy. Dual band units, like the latest Garmin Forerunner watches, can do much, much better.