A second running boom has hit, and this time it's a women-only party! Women runners can choose from a number of races in the U.S. exclusively for them, including the Nike Women's Marathon/Half-Marathon, which gives a Tiffany necklace to every participant. Women now make up more than half the runners in half-marathons in the U.S. and more than 30 percent of people who have been running for less than a year in the U.K. are women. If you're a woman, now is the perfect time to start running!