Exercising to the Beat of the Street

   Realizing I would be hitting a milestone birthday this year, I wanted to test my endurance and attempt my first half-marathon. A gorgeous sunrise over Arizona.    So last March I heard about Vacation Races, a company that organizes half-marathons near national parks, and found the Lake Powell Half-Marathon. An October race in Arizona sounded sunny and cool, and so I went to register but then waited, then waited some more until finally in September I summoned the courage and signed up to run 13.1 miles.          Flying on Oct. 13 to Phoenix, Kevin, Michael and I drove almost five hours to the tiny town of Page, Ariz., where we joined about 2,300 other runners. Our Knights Inn Hotel was perfect and conveniently across the street from the bib pick-up and race start at John C. Page Park, which eliminated so much stress. The only disappointment there was someone else's name on my bib. When I asked one of the staffers, she suggested that I just cross out the wrong name