You need to put yourself in the mind of the immigration officer who will consider your application for a longer stay visa. Their job is to encourage and enable non-permanent visitors who will benefit the country's economy by spending money while stopping permanent immigrants and/or people who will work.
There are three basic questions that the person considering your visa request will want answered and one possible supplemental one.
1 Is this request for an extended stay reasonable and believable?
2 Does the applicant demonstrate that they have the means to support themselves while they do their activities without needing to work locally?
3 Do they have a strong reason to return home when their visa expires?
Supplemental - Are they likely to require medical services while they are here and if so do they have insurance or some other way of covering those costs?
With this in mind then the more concrete your plans are, the more likely that you will get your visa.
E.g. Saying that you have not been to Spain for a while and you want to come and walk a couple of different Caminos, you are not yet sure which ones, and when you are finished you will go home.
Vs
Between X date and Y date I will walk the Camino Z from A to B. Rest three days in Santiago then between V and W I will walk the D Camino from C to B.
In addition you will probably need to demonstrate your Camino experience by showing where you have walked in the past and perhaps even get an endorsement from a credible local religious order.
You will need to show that you have sufficient resources and time to do what you say you will do. If you are not of retirement age then you might need to explain how you are getting this extended time off work. Saying that you are currently unemployed is NOT a good answer for this question of how you will get the time to do this pilgrimage (s).
You will probably need to highlight your family and/or other permanent connections to the UK and demonstrate that you have an ongoing place of abode within the UK, either owning a home in the UK or showing that you have a long-term lease or rental agreement.
You may need to show that you are currently in good health and that you have adequate health insurance to cover the period of your visit.
When you have this stuff, decide which country you will spend the most of your time in and apply to the embassy of that country.
Once you have done this then you should have no problem getting an extended stay visa.