Well, I'm a cold weather wimp myself, but I just did a 72 mile ride the weekend before last (Tahoe !!!) where the temp variation was as you describe - we started in the low 40's and warmed up to the low 80's. Coming from Texas, where a 'cool' ride of late has been anything under 100 degrees, I was worried about my body being in total shock from the temperature change. 40's is dead-of-WINTER riding for me. Anyway, here's what worked for me:

1. Keeping extremities warm to start. Toe covers (which I bought the day before) worked great - I'll probably choose them over my full shoe covers for riding here on most coldish days. Full gloves to start - if your hands are cold, you'll be cold. Both were easy enough to stick in a back pocket as they came off. I also, for the first, time, wore a liner under my helmet, which was fine (and small enough to stuff in same back pocket), but I also would have probably been fine without it. Ear warmers sound like a perfect alternative (and probably something I'd wear a lot in winter here).

2. I am a huge fan of knickers instead of shorts - I even wear them in the summer here because I learned I get a terrible heat rash on my legs when I'm out on a bike for 3-4 or more hours in full sun. But, for cooler (but not full-on cold) weather, they are hard to beat. Even with knickers, to start, I wore a pair of knee warmers under them. Again, a lot easier to peel off than my long tights. They kept my knees toasty until I was really warmed up and they also kept the lower part of my leg warm to start as well.

I spent a lot of time this summer testing out various brands of knickers. My favorites: Capo (have both light weight and fleecy winter ones - they are very flattering on and love the V-shaped wide waistband), Terry - Bella (also super flattering and comfy). Both have GREAT chamois.

3. Do you have a pair of warm arm warmers? After much experimentation, I'm a fan of arm warmers and a light (easily packable) jacket over a heavier jacket, again for portability reasons and that combo was fine for my start.

Have fun!