According to the Town of Aurora's website, the vision and foresight for the connection of trails came from Newmarket’s resident Mr. Ernie Crossland, who worked to make the trail a reality and a recreation treasure for the residents of all three Towns.
Earlier this month I cycled the Newmarket section (called the Tom Taylor Trail) with my family. It was a great ride and I highly recommend checking it out. Most of Newmarket's section is paved, while Aurora's and East Gwillimbury's are crushed limestone screenings (still very rideable).
Most road crossings go under the road, like this one at Mulock. The exceptions are at Davis Dr (bridge currently under construction, so there's a detour through the Tannery parking lot), and Water Street in front of Riverwalk Commons, which is a busy unsignalized crossing, so use extra caution.
West side of Fairy Lake. There are plans to pave it in the future. The route on the east side of the Lake is already paved.
Newmarket Riverwalk Commons - there's a great farmer's market here on Saturdays and many events throughout the year
We found that the trail had hills that were too steep for my 6 year old, so we put his bike away and pulled our kids on our trail-a-bike and chariot.
I found this trail flyer that covers the entire route: