Sounds like you had a great experience which is good! It’s very likely that the treadmill isn’t calibrated so the distance it’s measuring is likely wrong, and I don’t know about Apple Watches but Garmin and other sports watches have an impossible time tracking on treadmills without some kind of footpod. That said, there are also such things as beginner gains, and often beginner runners can pop out some surprising efforts (and then get a little cocky and go too hard too fast and get injured!). Do you think you’ll give it another go sometime?