Prepare for rain just in case. I walked the last week in September and the first week in October last year and it was perfect weather. A week after I finished there were torrential rains and flooding for over weeks straight. So you never know...
No way of knowing for sure, but do pack rain gear for this part of Spain no matter which you decide. It rains a lot, but it is a very green part of the country. (Even in early January there were flowers on some of the trees and shurbs.) I think...
Have you considered that your cycling might be the root cause of your problems https://www.cyclingweekly.com/fitness/lower-back-pain-dont-blame-the-bike-32094
I have a +/- 1cm displacement in the sacro-iliac and have lived with lower back pain for near 40 years. 6 weeks of novel exercise isn’t going to change a thing. I actually find carrying my properly fitted and loaded 7-10kg backpack a lot more...
Though we can give you general give IMO it is better to consult your GP / health practitioner or PT for detailed advise.
What works for me might be counterproductive for you .
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