Chelszilla,
I just had a similar issue. I had a 30 year old Columbia Blaze that was my first "right size" bike. Great bike. Really reliable on the FLAT - where I used to live. When I got to Central MO a few weeks ago, I tried to use it, but it was just so heavy that even with just my school stuff (I commute to work/grad school), it weighed 50 lbs and I couldn't get up these hills - they are seriously insane.
I thought about changing out the wheels and stuff, but it wouldn't do much good. That frame just weighed a ton.
Personally, if it was me, I would go out and buy a new to newer bike. If you are really lucky you will be average height and able to fit something used that is in great shape. I got SUPER lucky and found a tiny bike that had 50 miles on it - my Giant OCR. It has a carbon fork and an aluminum frame and is just so light.
Good for you for doing a tri! You are braver than I and if you can do it on a steel bike, you can do it on anything!