I am getting back into running and decided to use my Garmin Forerunner 110 from 6 years ago that I only used on about 25 runs. Amazingly, probably from low prior usage, it still works and holds a good charge.
I get connected to the GPS satellites only a few moments prior to starting the run. The first .1 miles is a bit wonky with pace (recently had me at 2:52 pace for 5 or 6 seconds) and the actual map you look at on Garmin Connect after you upload your run data has my actual starting point a good 100 meters off. It gets more and more accurate over the next half a mile. After that it's pretty good. It MAY be more accurate the first mile if you spend more time with the GPS on before you run (say doing warm up stretches), can't say for sure.
The 1st mile due to the initial discrepancies is usually about 7 or 8 seconds off, but the miles after that are within 1 or 2 seconds. Pretty solid stuff.
Google Maps is extremely accurate IMHO. The best way to determine this is to use Google Maps to measure a verified running track at your local college or high school and simply measure Lane 1, taking care to measure just outside of the inside line as that's where they measure the track lengths. I've had the measurements for the track come back as 399.5 to 400.5 depending on the finesse of the measurement, but we're splitting hairs at this point.