From Barcelona to Lisboa

Hello Fellow Explorers,
I'm about to arrive in Barcelona and my idea is to cycle to Lisbon.
I've thought about a few options:
1) Cycle to the camino del norte all the way to finisterre and then down to lisbon
2) Cross the centre of Spain above Madrid
3) Cross the centre below Madrid along the Tajo river all the way to lisbon
I dont mind mountains or flat, unless it's all the way mountains (ups and downs for 1500 km...too much!) or all the way flat (boring?).
I would love to cycle on secondary roads, surrounded by lush vegetation and no traffic, but I guess that's everybody's dream!
I'm pretty independent and on a very tight budget (less than 4 euros per day)...
I'm riding a touring bike and I've already done around 10 thousand km starting a year ago, from italy to thailand...
Any suggestion?
Thanks