I think your first goal should be to finish and to have fun! With that in mind, there is nothing wrong with getting base miles under your butt and feet. A lot of it depends on the level of fitness you enjoy right now.
You should start off slow and do not add more than 10% to your mileage each week. You should be keeping some sort of journal as to distance/time you have completed. Be sure to take rest days.
Some programs have you completing duathlons with 12-13 weeks of training prior to your event.
See this link for an idea of training. It is only 8 weeks, but I beleive it is for someone with some fitness under there belt. But it gives you some sort of idea of what you should be doing.
I have a coach who tells me what to do - I don't know if I'd be as motivated if I wasn't accountable to someone. But that's me.
There are tons of people that are successful at completing this type of training on their own.
Go get em!
http://www.team-magic.com/8_Week_Lon...e_Duathlon.pdf
"You can't get what you want till you know what you want." Joe Jackson
2006 Cannondale Feminine/Ultegra/Jett
2012 Trek Speed Concept 9.5/Ultegra/saddle TBD