Hello and welcome!
Before I did a tri, I did a duathlon. I wanted to get a feel for moving from one dicipline to another and for the transitions.
It was a nice way to enter the sport.
Although, I have to say... I am now sworn off du's. I'm such a horrible runner... when I do a du, I start at the back and never really gain any ground from there. At least in tri's, I'm fast in the water... so I come out early and then just get passed on the bike and run. At least I get other racers around me though. I have found that I don't really like racing alone out there.
Enjoy your du! If you are a runner, you will enjoy it.
As far as the bike-run-bike combo. It's odd. Never saw a du like that around here. Although, I would pick that over run-bike-run! I'm sure it's just something the race director selected to do.
Good luck and let us know how it goes!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"