Hi Donna!
Based on what I've seen at organized running events, if it's "dangerous" to run at your weight, you'd be in good company living dangerously because there's just about EVERY body shape out there.![]()
I have no idea about the specific knee/joint issues, it makes sense, but 1) your legs, especially your quads, are probably very strong if you've been cycling and walking regularly and 2) if you increase only very gradually you will give yourself a chance to adapt.
It can't hurt to loose a bit more weight but also realize that running will make you hungry (like cycling). That may be one other thing to think about...
I strongly recommend that you follow a walk/jog plan that emphasizes walking. If you search the Internet a bit you will find many "First 5K" plans, like this: http://www.runnersworld.co.uk/news/article.asp?UAN=67 It gives you a ball park figure, but I would recommend that you do not increase the run times quite as fast as they recommend. You can write your own program! I find it motivating and generally easier to plan ahead and, on the day of the workout, just do "what's on the paper."
Sorry for being so chipper, I get really excited when people say they are interested in picking up running! I HATED running from the bottom of my cycling heart three years ago and now I actually look forward to rainy days so I can go for a run.Feel free to join the running thread, too!
Enjoy! But DO take it easy.