I started the C25K this October, and I was 185 (5'-4"). I had been walking for 2 years, mostly on an asphalt track, and after the first 2 weeks of C25K I moved indoors to a treadmill. I've experienced some joint pain, but I believe it's related to a car wreck in 2006, not my weight. If you are otherwise healthy, try the first week and see if you feel any discomfort. I've always heard that sidewalks are the worst for your knees, tracks/roads are not much better, treadmills are better, and smooth trails or grass is the best surface to run on. The BEST thing about the C25K is that from October to now, I'm down 20 pounds. It took me two years to lose 15 pounds by walking a 5K every morning, and now I run a 5K in less than 40 minutes every other day.
1000 miles in 2010 - 7/1000