I'm so glad to read that advrider isn't just about dirt roads..!
If it is your first, try to make it memorable. not a 500 forward and back.
That's what I will do anyway!
Ideally, pick fairly fast roads that you never rode. Or if safety is an issue and the unknown is not pleasing to you, try this: pick a road loop that you might have done in one direction, over a few days in the past, then ride it in reverse. Its not quite the same road, but you know its pace and traffic, and gas spots! Set a time table to avoid rushhours in urban area, if any.
I will try it my first ss1k next summer. I already did 1300km in a day, so I feel I can give limited amount of advices. I rode the intended roads already in one direction last summer. This time I will do the other direction.
FYI, for best conditions, I would hold on til june or july, for more day light, more warmth in evening, and less low sun. Meanwhile, I would do the loop counter clockwise, trying to keep the sun in my back most of the day. Besides, landscapes are nicer (greener) in june without backlight and without shadows on the roads, but that's just my opinion...! If you can leave at 3am, and breakfast with sunrise on some nice mountain view, you will never forget this 1st ss1k!