Hello! I am a cyclist, and I am not slim, but I happily wear Lycra, larceny because it is practical and comfortable.
Is it a Breeze ride you have signed up for?
For 10 miles, you really want to think about some padded shorts, even if you wear leggings on top of them (and you should go commando underneath). Sports Direct, Decathlon, Halfords and Aldi (a few times a year) all sell budget padded cycling shorts. (Also rub the padding with Bepanthen before hand, then sudocreme on sore bits afterwards). With cycling shorts, yiu di get what you pay for to some extent, but those shops should sort you out initially. You can get padded pants to wear under ordinary clothes, but these are no cheaper, and not as effective.
On your tip half do you have a moisture wicking top from any other sports? If so, that would be good, Bynum a t shirt will be fine.
For shoes, trainers will be fine, but if you have some stiffer walking shoes, they will be best.
Also think about where you will carry things. Cycling jerseys have pockets in the back for waterproofs, inner tube, lock, jellybabies etc? Do you have a small rucksack for these things? A lightweight waterproof?
Presumably you have a helmet? If nothing else, beg, borrow or buy some fingerless cycling gloves. Yiu ca get them for £5-10. They’ll help with grip when you are sweaty, absorb some shake and protect your palms when you fall off.