I try to stay away from extracts if I'm going for a real rootbeer. Most modern rootbeers use wintergreen instead of sassafras because of "health" issues. I don't buy all of those issues...so I use sassafras.
Check out this recipe... It's a little more complex, but adding yeast to it should allow you to carbonate it fairly well if you don't have a kegging system.
First, put about 5 quarts of water into your favorite brew pot, and then add:
•1 oz sarsparilla root
•1 oz sassafras root bark
•1 oz cherry bark
•1/2 oz licorice root
•1/2 tsp freshly ground nutmeg
•1/8 oz cinnamon (about half a stick)
•2 to 4 oz raisins (you can add more if you like their flavor)
•1/2 tsp salt
•3 cups honey
•1/2 cup molasses
•4 lb sugar
Simmer this mixture for an hour or so, until you have a 5 quarts or so of syrup left in your pot (remember, adding all that stuff increased your volume by a couple of quarts).
Once this is boiled down, strain your mixture into another pot and add:
•1 tbsp vanilla extract
•1 tsp wintergreen or birch extract, or 1/2 tsp pure essential oil (optional)
Stir a bit, and then bottle in 1-quart jars. Due to the high sugar content, this should keep fairly well, but you could always use a plastic jar or bottle and store it in the freezer (do not put glass into the freezer). To make the root beer, just mix 1:4 with water (one part syrup + 4 parts water) and carbonate4. Depending on your taste preference, variations in cooking times, and/or the spices you used, you may find that the ratio needs to be adjusted for your batch, so don't hesitate to use more or less syrup to get the flavor just right.