Congrats on your first commute.
Some people can ride short commutes in their work clothes and not work up a sweat...I am not one of them, so I carry my work clothes (pants and a shirt), which fits easily into my rack trunk, along with my lunch. I keep shoes and belt at work, as well as a complete set of clothes, just in case Murphy decides to f with me.
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"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey