The trail is maintained in different sections by the Seatle partks dept and by King County Parks and Recreation dept. As far as mile markers, I haven't payed much attention to them recently since I have a computer on my bike. But, to the best of my memory they are irregular at best. The markers that are there are in miles, for the most part. Since the trail was built a section at a time, they don't start at the actual beginning of the trail and don't continue to the current end of the trail.
There are a couple of sets of mileage markers starting at Marymoor and continuing into the U district. One set are posts starting at Marymoor - unfortunately they removed the Mile 0 post and the distance betwen the mile posts is a little short of a mile. The other set are yellow numbers that are painted on the asphalt - they seem to be more accurate but the trail has been rebuilt so many times, you can never be sure the distance is correct.