It's not long distance walking. It's walking a short distance and have a coffee. Walk another short distance and have a coffee and croissant. Walk another short distance and have a light lunch. Walk another short distance to your bed for the...
Seville has many bike shops and several which specialize in used bikes (Google 'buying a secondhand bicycle in Seville'). This would give you a better choice than the take-it-or-leave-it of buying privately. You could take your own pannier bags...
None of them are difficult, if you are referring to climbs or distances. The final one is pretty boring because it is basically walking through ex-burbs to suburbs into the city.
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