Really difficult to say whether 18mph is a good pace, especially in the context of a triathlon. Wind is also a factor. For example there's a short segment that I do, and I can push 16mph against a 7-10mph wind. On the return leg with tailwind I average 20mph with what feels like way less effort. I don't know if I have it in me to get to 20mph against the wind and sustain it. I've also never had 12 miles of uninterrupted flat road to try these sorts of tests.

In general though, aerodynamics make a huge difference, regardless of the rider's body type. The goal is to get to a position that reduces the frontal area as much as possible while still being able to tolerate it from a comfort perspective, and maintain strong power output to the pedals.

A dedicated tri (or TT) bike will certainly be more aero and help you ride faster but there's a trade-off in comfort. But if your TT efforts are short that may not be a problem.