I read here somewhere, and tried it and it works for me: PB&J sandwiches. They have carbs, and protein, and both fruit and refined sugars. Plus they're yummy and cheap. Works for me.
BTW if you've been commuting for a while I don't think doubling the days will be a big stretch for you. I recently rode 70 miles, when I normally ride 11 miles twice a day, and had never ridden more than 28 miles at a time before. It was not a problem and I was not at all sore the next day. Once you get conditioned, if you take it easy, you can do a lot more than you think you can.
And if it does hurt (sore, not injury hurt), remember what you feel is you getting stronger.
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Work: the 8 hours that separates bike rides.