I took some benylin (dextromethorphan) extra strength that had already expired. What is this going to do to me?

Nothing bad. After a certain period of time, drugs lose potency. The rate at which a drug can be expected to "expire" is generally when it reaches 90% of labeled strength. A very few drugs, especially "biologicals", may become toxic. Check with your pharmacist about this. Benylin's ingredients don't become toxic as they age (provided storage requirements such as temperature, sunlight & humidity are maintained).