Tucson offers the largest concentration of Mexican eateries in the United States crammed into an area dubbed "the country's best 23 miles (37 km) of Mexican food.†Putting on our stretchiest pants, PAXnewsWest.com dove into the Tucson food scene with Visit Tucson to sample as much as we could in 24 hours. A tour is the best way for visitors to maximize the time (and calories) invested; we rode with Gray Line Tours Arizona on their Best of the Barrio tour, offering five stops in Southern Tucson…