How's that New Year's resolution to exercise coming along? It's been a few weeks since mapping the city's yoga studios, figuring out the fittest neighborhoods and visiting Racked DC for their Fitness Week. Still interested in staying in shape now that it's the third full week of January? Perhaps the idea of working out under your own roof sounds more appealing. Here are a few houses in the greater D.C. area with exciting and extensive home gyms that are currently on the market.