Al, I've ridden two or three 200k brevets or permanents in jeans, with tighty whities underneath, not cycling shorts. Just depends on what it takes to make you comfortable.
I've used lots of non-cycling clothes and am gradually transitioning to cycling clothes. Some of it is sort of indifferent, some is really good. I really like my cold-weather tights, for example.
Leg warmers do come in handy, and one drawback to gym shorts is you can't conveniently use leg-warmers with them.
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"be careful this rando stuff is addictive and dan's the 'pusher'."