I did it last year in September and October. The first ten days out of Sevilla it was furnace-like: high 30s, even 40+C. You could not possibly walk in the afternoons. Then it rained a monsoon-type rain. After that it was bearable - sunny and pleasant, but up towards Zamora in October the mornings were pretty cold. Once I turned west along the Sanabrés it rained and rained, and then in Galicia it RAINED AND RAINED, and there were floods. This doesn't mean to say you'll experience anything at all the same, except it most certainly will be hot in Andalucia and southern Extremedura.