For those that don't think an a person can be blindsided, even if alerted, remember cases like Amber Deboise, Chelsea King, and, from my own memories, a friend who fought off an attacker while running and ended up with a only small cut on her neck, and my daughter running in broad daylight in a large city on a busy street, who fought off an attacker while at high noon.
It's probably always better to run with a companion or carry pepper spray or another means of protecting yourself....and, as stated by others earlier, don't have earphones on both ears!