Alright, here is the best route I can think of: http://maps.google.com/maps?f=d&saddr=248+Blakeslee+Hill+Rd,+NY+14867&daddr=horseheads+ny+to:pleasantville+ny+to:white+plains+to:41.022391,-73.821945+to:I-495+S%2FI-95+S+to:I-495+W%2FI-95+S+to:5000+Seminary+Road+Alexandria,+VA+22311&hl=en&geocode=%3B%3B%3B%3B%3BFQYlUAIdtEtp-w%3BFUATUAId8qpn-w%3B&mra=dpe&mrcr=3&mrsp=4&sz=12&via=4,5,6&sll=41.016174,-73.857307&sspn=0.189621,0.286674&ie=UTF8&ll=40.962012,-73.790359&spn=0.189777,0.286674&t=h&z=12
If you want the absolute fastest route (I changed your location for D to be the White Plains train station to be more accurate) to the GWB, this should do it: http://maps.google.com/maps?f=d&saddr=248+Blakeslee+Hill+Rd,+NY+14867&daddr=horseheads+ny+to:pleasantville+ny+to:28+Hamilton+Avenue,+White+Plains,+NY+10601+(White+Plains+Station)+to:41.014879,-73.790016+to:5000+Seminary+Road+Alexandria,+VA+22311&hl=en&geocode=&mra=dme&mrcr=3&mrsp=4&sz=12&via=4&sll=41.029384,-73.737488&sspn=0.189583,0.286674&ie=UTF8&ll=40.988451,-73.747787&spn=0.189701,0.286674&t=h&z=12
The only thing I should note here though is that for a stretch of a good 5 - 10 miles, the Bronx River Parkway is easily the most dangerous highway to drive on at higher speeds. It is often a 4 lane highway, and there is no safety room between the left and right lanes and the guard rails, coupled with the additional twists and turns you have to make, you have to be an excellent driver if you want to drive faster on that highway, especially if you run into traffic there.