Why not? From the looks of things, if he starts pretty close to the mouth of the river (onto the lake, that is), which appears to be so, it should be at LEAST 30 seconds to get down to Riverwood.
Well what I thought:
He starts roughly where the red line of the border crosses the path from Helgen to Riverwood. He walks north to riverwood. At the bends in the road on the hill he encounters the Standing Stones (hills directly south of Riverwood). He then continues down the same path, adjacent to the river, to Riverwood. Where he walks on a road next to the river for a good 30 seconds if not more and finally arrived in Riverwood.