Why not start in Brewster. The trail now goes to around 1 mile from the station there. You could ride the trail south to Eastview, which would leave you close (and downhill all the way) to the Tarrytown train station. Problem is your total mileage would be closer to 40 than 10-15, but you could get in a nice rest stop at Yorktown Heights at around the half way point. FWIW from Mahopac to Carmel the trail is all wooded and you don't go though any towns if I remember correctly. The section from Yorktown to Millbrook is very nice as it takes you over the Croton reservoir. From Millbrook south to Eastview there is a short stretch where you have to ride on Rt 100.