Your situation is very common on Oregon rides. Even in warmer months, it's always cool overnight, so we often start in cool temps.

Armwarmers and a light vest are two of the most important things you can have in your wardrobe. Armwarmers can be pushed down when you aren't quite sure you're ready to remove them. Once you do, they roll up like a pair of socks and fit easily into a pocket. Vests can be had for as little as 3 ounces. These things scrunch down to something the size of an egg (seriously) and can be put into a pocket, a seat pack, or smushed into a teeny little stuffsack (or it's own pocket) then stuffed in a pocket.

If it's really going to be 75 with no chance of rain, I'll usually forgo the leg warmers and just have cool legs the first hour or so.

Susan