You know, you might double-check that advice with a sports medicine doctor. While it's true that running can be tough on the knees, that can be at least partially countered by correct training, form, shoes and other factors. Have you actually had knee problems?

Is the Iron Girl tri distance the 0.62mi Swim - 17.5mi Bike - 3.3mi Run one? (I Googled Iron Girl Triathlon & found that.) 3.3 miles should not be a punishing distance to train for or complete.

If it were me, I'd get a second opinion from a doctor that is more familiar with body mechanics. I'm sure the thyroid guy is very smart, but knees and running and all that are not his specialty.

Good luck!