Resist the temptation to run like hell while she is in the bathroom.
If you really need to cut the date as short as possible so you don’t stab yourself in the eye with a fork, summon the waiter while she’s in the john. Give him your credit card, ask for a 20 percent tip to be added to the check ASAP (to spare you the back and forth of adding a tip), and explain that you have to leave because of an emergency.
When she gets back, apologize profusely and reveal that you just got a call informing you that your bathtub is overflowing and leaking into your roommates’, neighbors’ and landlord’s spaces. (Of course, you’re making all this up -- only Hugh Hefner’s bathtub would cover this much real estate.) Any variation on such a possible, imminent, but not fatal, disaster is fairly acceptable. If you look agitated enough, she’ll be distressed for you and hopefully help hasten your departure. Thank her for being understanding and let the host know it was nothing about the venue/food -- just a personal “emergency.”
In the future, learn to prescreen better. For blind dates, in particular, trade more photos and engage in at least one reconnaissance phone call. And never trust your mother’s hairdresser’s roofer’s best friend’s cousin to set you up.