The suggestion for parallel streets instead of huge busy roads is a great one. Googlemaps is also a wonderful tool, but as others have mentioned, its not a complete work. It's hard to see elevations on it, and if unfamiliar with the roadway traffic, you could end up with lots of cars. A detour is a perfectly viable option. You could also perhaps find a sympathetic coworker who might be able to drive you partway, or maybe take a bus partway? That's never worked for me yet, but it doesn't sound too complicated.

If your route has reasonable bike lanes, uncomplicated sidewalks, or other accommodations, you can simply take the time to acclimate yourself to heavy traffic slowly. In addition, once you get out there it gets much easier, and you might start encountering the same car commuters who will figure out quickly how to get along with you. If, on the other hand, you would be out in the thick of irritated motorists without adequate room, I'd say that you shouldn't attempt to get used to that type of riding, since its only a matter of time and statistics before we read about you on the news.
For the freeway, are there any light-controlled intersections you could use, or over/underpasses?

Sorry I can't be more help, but I completely understand your desire to get out there on your bike. You have even more guts than I do to attempt 16 miles at first.
I hope you figure it out!!