Torps, since you're coming into white plains via 119, you
might be better off cutting up to route 100C from somewhere along your route, rather than starting up Virginia Road/Grasslands as I was suggesting. My way has a somewhat annoying hill at the beginning - there may be a way to circumvent it coming from 119. I don't know how ridable 9A (Saw Mill River Road) is, but that goes from 119 north to 100C, and then you're very close to the entrance to the North County Trailway.
Anyway, here's the route I was originally talking about, from the North White Plains station, following the Bronx River Pathway up to Virginia Road -
http://www.toporoute.com/cgi-bin/get...AQWGBKUBWEGVOK
But if you zoom out a bit, you can see that 119 parallels the route, so you could potentially cut from 119 to 100/100C on either 9A, 100A or 100.
good luck!