Caramel Chex Mix

In my Thanksgiving menu, I mentioned that I was going to make this to snack on, and of course, I did.  But when I made it, I was reminded how little I like measuring out 9 cups of cereal...in fact, I refuse to do that.  Why does Chex make recipes that don't just allow us to dump the box of Chex (which contains about 13 cups), instead of measuring it out?  So, I've posted their recipe modified, so I can use the whole box, well, both boxes actually, and use a saucepan and oven instead of the microwave.  Warning, this makes a ton, and you will eat it all! Oh, and of course, it's gluten-free, dairy-free and soy-free!

Caramel Chex Mix

1 box (~13 cups) Rice Chex cereal
1 box (~13 cups) Corn Chex cereal
3 cups packed brown sugar
1 1/2 cups Earth Balance soy-free margarine
3/4 cup light corn syrup
3/4 teaspoon baking soda

Directions
1. Heat oven to 250F.  Spray a couple of large cookie sheets and/or roasting pans with cooking spray. Dump cereal into pans.
2. In a large saucepan, stir together brown sugar, margarine, and corn syrup.  Cook and stir 8-9 minutes over medium-high heat until mixture starts to boil.  Boil 1 minute, stirring constantly.
3. Remove from heat.  Stir in baking soda. Pour over cereal mixture, stirring until evenly coated.
4. Bake 40-50 minutes, stirring every 15 minutes, until coating is set.
5. Spread on waxed paper to cool, about 10 minutes.  Break into pieces and store in an airtight container.