Because we have a magnetic field on Earth, the Hydrogen in our atmosphere doesn't get lost to space like it did on Mars and Venus. There, the light elements float to the top of the atmosphere, the sun ionizes them and the solar wind sweeps them away. That's why Mars and Venus lost all their water. At earth, the ionized hydrogen is trapped by our magnetic field and much less can escape to space (only over the poles). The solar wind travels very fast - from 200 to 600 km/s under normal conditions, and can be even faster during major events. If the orbit of Mars and Earth were circular, Mars would be only .52 AU away from us (78 million km), so at 300 km/s, it would take us only 260000 seconds. There are 86400 seconds in a day, so that's 260000 seconds = 3 days. But the solar wind can blow faster, and if Mars is at its closest to the Sun when we are at our farthest (as it was in 2003), the distance is only 54.5 million km. At 600 km/sec that's only 90833 seconds, or just over one day.