"Tri" bikes aren't necessarily faster than a road bike. Things like course elevation, distance, and conditions are all factors to consider when looking for mph.
I am 45 years old over weight by 10 pounds. I ride a Trek Madone with a regular helmet, other than that, how can I go 1 mph faster?
More aerobic training + better nutrition. Drop those 10 pounds. Once you're in shape, find someone with a lot of experience fitting riders for tri setups (preferably someone F.I.S.T. certified) and go from there.