I just wanted to throw my $0.02 in about exercise:
I started running when I was in the mid-170s. Now I'm a little taller than you are (about 3") but not by much. I was careful, I didn't leap right into it. I used the Couch to 5k program, which starts you off with 60 secs of running at a time, interspersed with brisk walking. IMO, as long as you're reasonably healthy and don't already have joint issues, you should try it. The worst that can happen is that you find that it isn't working for you and you go back to walking.
But as Steve said, there's more than one way to walk as well. Pumping your arms, changing your stride, etc ... walking is more than just strolling.

You can do all kinds of things to make it work for you. And there are lots of other exercises you can do as well. Be sure and read the posts in the exercise area that are sticky'd to the top. There's good information on weight lifting and body weight exercises that you can do that will help raise the intensity of your workouts w/out necessarily raising the impact on your joints.
I will also agree with Steve that food is the most important part of losing weight. It was pretty easy for me to lose just by cutting back when I first started. But after a while I really had to pay attention to what I was eating.
I'm dealing with that right now, in fact. I am trying to move out of maintenance and back into moving and I find I have to be MUCH stricter with my food - not just how much, but what. Before a few slips each week wasn't any big deal. Now ... it can make a difference.