Maybe you’re a solitary walker. Maybe you have a pal you meet on the corner every morning, or a group you meet at the mall. We’re guessing, though, that you’ve never walked with thousands of other people. You’ll have your chance on Sept. 8, when the North Texas Heart Walk celebrates its 25th year. Had you done this in 1993, its first year, you’d have shared the sidewalk with 450 other walkers. That year, $50,000 was raised, a number that now tops around $6 million every year.