Well, I don't know that there is a way to do exactly what you are asking, and I can probably see why. If you just started the program and made any changes to your trip, you'd have to recalculate it anyway. If you opened the program and have not made any changes, then this is what I do, and it may help you.
I realized a while ago that when recalculating, most of the time is spent in figuring out road connections, etc, and then "some" time is spent preparing the itinerary. I also saw that that there has been a option there, for the past few years, to select whether to do a complete recalculation every time, or not. So this is what I do.
Load my trip, plan, calculate, recalculate, modify until I have what I want and the calculate it one more time. Then, I make sure the "Complete recalculation" option is NOT checked and save my trip file. I then close the trip dialog, go to File, Program Options, go to the last Tab "Other Options" and make sure "Save Current Trip on Exit and Restore" is checked.
Now, whenever I start the program, my trip file is already loaded and my route highlighted. I open the trip dialog and click Get Directions, but since the road selections are already there and saved with the program, it only takes a few seconds to get my itinerary. I then click on the little paperclip icon which anchors my itinerary out of the way (but still on the map), close the trip dialog and I am good to go.
Hope this helps.
BTW, if you have any improvements in mind, that you'd like to see, contact undertow directly. They are pretty responsive (most of the time <g>).