Hi MeganB! Congrats on training for your first duathlon as a type 1! (I did my first tri 3 months after my diagnosis).
This is my experience: I've learned to wake up about 4 hours before my event. Eat a good breakfast (like oatmeal with fruit) and bolus the normal amount. Then I go back to sleep if I don't have a commute to the event. I will check my BG 30 min before the event, and if it's lower than I'd like, I'll eat a snack _without_ bolusing. I set my basal to 4-exercise (animas pump), which is programed to go to -50% 1 hr before the start of the race. If it's a long event the basal rate will increase to -40, -20% successively to account for the carbs I'm taking in.
As far as dexcom and testing goes, I would definitely test at T1 and compare to your dexcom reading. If the arrow is flat and the numbers are relatively close I will rely solely on my dex, if the arrow is going down and there's a discrepancy b/w dex and the meter, I'll eat than probably stop to check halfway through the bike, just to make sure my BG is at where I really think it's at. And of course test again at T2. Also prior to the event during training, test, test, test. If you stop during training and learn how you respond to various exercise intensities, you'll have a better inkling race day. (Not to say that diabetes doesn't sometimes throw all the rules out the window).
A great resource to look into is TeamWILD. I have an olympic triathlon in 2 weeks and have been training with them. They're amazing and wonderful and know their diabetes training! http://teamwildathletics.com/ Working with them has taken most of the guess work out of endurance sport and diabetes for me. The more I know and prepare, the less anxiety I have about BGs. Another group of active diabetics can be found at insulindependence.org.
Good luck on your Duathlon, and let us know how it goes!