I work in this area and I was exploring. Some of the runs are pretty long for Toronto, If you did a straight bomb on york mills im pretty sure you could hit 65kmh.
Graydon hall Dr. is a nice long run thats not too steep, but it does end at don mills road and it can be busy at times. Its fun to cruise down at night.
All the other runs north of York Mills are very low traffic and are great for freeriding.
Valley Woods Rd and Laurentide are nice mellow runs.
You should atleast know how to footbreak of your gonna hit any of these.