Molly - I started running when I was at 215 pounds or so, and completed my first 5k when I weighed 205 pounds. Still obese for my BMI. And you know what? There were lots of people who came in after me. I am sure every race is different, but if it the kind of race where people are encouraged to walk as well as run, and you try to run at least some of it, you are pretty much guaranteed not to come in last. In my race, there were several people who took over 60 minutes to complete the 5k. There were people of all ages and fitness levels in my race, and I imagine that is the case in most races, especially ones like yours that are for a cause so people participate to support the cause, not just cause they are atheletes. And, okay, I bought the picture the race photographer took as I crossed the finish line - and guess what? I am finishing right in front of skinny minny runner - if you train well and prepare, it is possible to be faster than others who may have a lower BMI but aren't as prepared. Not that I encourage being competitive, but, I gotta admit I love that finish line photo!
I agree with Rochester - it can be tough to overcome bad gym class memories. Mine include being the last to finish the mile, running/walking 4 laps around the track and wheezing, red-faced, with sore muscles for days afterwards. Now, my mile
PR is 8 minute 30 seconds (almost twice as fast as I did in high school) and I regularly run ten to fifteen miles a week. Your past doesn't define your present, or your future.
I ran 5k on the treadmill quite a few times before I tried to run in outside, and before I raced, so I felt confident with the distance, and I'd recommend that. Also, keeping in mind that running is high impact, I would recommend doing other exercises on 'off' days - I run 2-3 times a week and do strength training, yoga, swimming, dance classes, etc on other days.
Congrats on your loss so far! Keep up the great work. Let us know how your progress goes - we are all here to cheer you on.