Yeah, it's tough for me, too. However, you seem infinitely more knowledgeable about the type of bike you want than I was. You'll be great.

If I were you, I'd go to the Trek dealer nearest you and ask to see the model you're looking for. Take an opportunity to say why you are looking at that bike over a different one. It is likely that if they don't have one in stock (you could call first to find out), they will try to get you on one they have lying around. If you can, narrow down what the Trek has that you really want, and ask if that bike has that feature. If they can't tell you why that bike is better for your needs don't bother riding it.

OTOH, there may be a bike you haven't considered, so don't close the door completely to new bikes. It might just be "the one".

The other thing...be up front about your stature. I guarantee you the young shop guy (it's more likely a guy) is already feeling uncomfortable because he doesn't want to say anything to offend you. Disarm him if you can by mentioning it first. My husband is extremely tall, and let me tell, you some shop people are uncomfortable even mentioning THAT in context of a bike (erm, they don't even MAKE bikes to fit him).

Bottom line, if you go in as an educated consumer, aware of the obvious facts, and let them know, you are likely to get farther and get better help. Be your charming self and you will be fine!

Karen