Maybe browse around Strava.com to see if any other locals commute or ride along the routes you're thinking of using. I only live 5 miles from work but there's about 500ft of elevation between the two and a mile long hill with 10% grade. I looked at the maps on Strava and found that many riders take that hill, but I need to be comfortable hitting speeds of 30-40mph on a bike with cars coasting up to 50 (limit is 35 on the stretch). Armed with that I decided I need to do some climbing training (if I want to get home!) and really learn handling on my road bike at high speeds before I go barreling downhill to work.
2011 Trek Lexa SLX "Silver Surfer"
1998 Specialized HardRock AX FS
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!!
My full commute to work is 28 miles and takes me through 6 towns, on country backroad (hills!), skeevy neighborhoods, strip-mall highway, suburb, bad-part-of-city, and city streets. I start in the dark and, on the way home, end in the dark. It takes me two hours one way. But that two hours flies by. It is one of the most interesting ways to see where I live, and I enjoy it very much. But I don't do it every time. Sometimes, I ride to the halfway point where there is a park-and-ride, and do the more reasonable (and flatter) 14 miles in. Other times, I ride to the bus (5 to 12 miles from the house, depending on which bus I take) and put my bike on the rack and ride. When I first did it, I chose a Saturday and did the full thing, and got really lost, and had some fun with it, and mapped it when I got home on mapmyride.com When I finally commuted with traffic, it was hairy at first but after awhile you really do get the hang of the flow, and how to be visible, and communication with drivers (mostly good), and packing a backpack, and all that good stuff. I guess I just wanted to say it is DOABLE and the best way to do it is to do everything above and just go for it. Give yourself extra time, and be willing to get off the bike if you have to or change your plans. Have a backup plan. Keep trying.
I can do five more miles.