Based on eye-measurement from the map of Varisia in the Rise of the Runelords Map Folio, I'd say there's approximately 250 miles (direct line) between Sandpoint and Turtleback Ferry.
The most convenient travel route would be a lot longer, though (travel to Magnimar, then boat on the Yondabakari River through Wartle, Whistledown and Ilsurian, and finally on the Skull River to Turtleback Ferry). That route would be approximately 400 miles.
There are several mountain ranges near Turtleback Ferry. The Iron Peaks and the Wyvern Mountains both come close to it; their southern ends are about 15-20 miles to the north (Iron Peaks) and north-east (Wyvern Mountains). The Malgorian Mountains are about 80 miles to the west. The Fenwall Mountains are the only mountain range south of Turtleback Ferry, but it's around 100 miles from the city.
The RotRL players guide is a free PDF. The area map is the last page and has a scale you can use to measure distance.
http://paizo.com/products/btpy8bd9?Pathfinder-Rise-of-the-Runelords-Players -Guide
My players made the trip on horse back:
Sandpoint to Nybor (90 mi. @ 48 mi./day = 2 days)
Nybor to Wartle (25 mi. @ 48 mi./day = 0.5 days)
Wartle to Whistledown (85 mi. @ 48 mi./day = 1.75 days)
Whistledown to Ilsurian (60 mi. @ 48 mi./day = 1.25 days)
Ilsuiran to Turtleback Ferry via Pendaka (80 mi. @ 48 mi./day = 1.75 days)
Turtleback Ferry to Ft. Rannick (20 mi. @ 48 mi./day = 0.4 days)