Hi mikerod76! I don't think this is a bad plan at all. November isn't considered a "PEAK" travel season in Spain so your chances are good to just show up and find something. Still, when you check-in you'll have to give them a check-out date just so they can make future reservations. No hostel owner wants to take the chance of leaving a room un-rented even for a day.
Still, giving a quick call to those hopeful hostels from the bus/train station upon arrival could save you some leg-work if you're carrying much. On the other hand, there's something to be said for seeing the neighborhood even before making a reservation and committing yourself. It'll be November so carrying your stuff around won't be too uncomfortable/hot. (at least in Madrid... Sevilla will be a bit warmer)
I URGE YOU to do the courteous thing and CANCEL any un-used reservations. If not by phone, you can do this from the portal/door by buzzing and giving your name. It's only fair and "the right thing to do".
Saludos, MadridMan
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