A Song For Arbonne by Guy Gavriel Kay

Opening line:

On a morning in the springtime of the year, when the snows of the mountains were melting and the rivers swift in their running, Aelis de Miraval watched her husband ride out at dawn to hunt in the forest west of their castle, and shortly after that she took horse herself, traveling north and east along the shores of the lake towards the begetting of her son.