Kathy,
I started at the beginning of Apr, and while there wasn't much rain, it was still cold in the Pyrenees and on the meseta. I don't think you need 'several' layers of thermals, but you do need sensible layering. My approach was:
- base layer of a polyprop singlet and trekking shirt,
- middle layer - mid-weight fleece (half zip)
- outer layer - lined goretex rain jacket.
I started most mornings with the base and middle layers, removing or adding layers as needed. On only one morning was it so cold that I had to stop and put on an outer layer for insulation. If I had needed another layer, it would have been my spare shirt or singlet (and that would have involved stripping off the outer and middle layers before being able to add anything). I was lucky it didn't come to that in the end.
I wore long johns on the first day (from SJPP) and got so hot that I had to get out of them by mid-morning. I didn't bother again, but would wear gaiters if it got windy and I wanted to protect my legs, or wore my rainpants if I needed additional protection. This allowed me to add or remove layers on the road, unlike the long johns, where one needed somewhere at least a little private to change out of them.
I prefer to trek in long trousers, and having used zip-offs for one trip, now don't. These are personal choices, and you need to make your own. Just avoid cotton if you can - a light to medium weight modern quick drying fabric would be a better choice.
If you get your layering right, the question of a seasonal focus becomes less important. Unless you are walking in the depth of winter, good layering will give you the flexibility you need while keeping your pack weight down.
Regards,