Perfectly fine to be at the starting line at that weight - I was about 16 for my first tri and was very conscious of being the biggest chap there.. However, I finished ahead of quite a few of the munchkins, which made me feel better
I'm now progressing toward munchkin size myself - 14st now and heading for 12st12.
I know one thing for certain - the less I weigh the faster I can go - and it's certainly cheaper than buying lighter bits for the bike - just think how much you'd have to spend to take 5lbs off the bike (heresy, I know... I'll hang my head in shame later).
After years of yo-yoing with weight, I've found something that works that I've never tried before. I always thought it was the preserve of women - calorie counting! It works and I'm 37 pounds lighter for it this year!
I use Livestrong.com (myplate) to track all the food I eat and manage my calories - I've stopped tracking exercise, cos they overestimate what you use..
500 calories less a day = 1lb of fat loss a week = Job done! And it's relatively painless.
Good luck and smile every time you pass a thinny

Can't swim, can't ride, can't run
Doing IM WALES to raise funds to keep my daughter mobile:
http://www.getbeatricemobile.orgCompressport UK - Official Compression Supplier to sonofsammo