There is a bike rental company in Seville. You can ride bike to Santiago from Seville.
Drop it off in Santiago. Or they will pick it up from your location in Santiago.
Many good comments about this rental company in Seville (Triana):
http://www.trianahostel.com/en/bike-rental-st-james-way-camino-de-santiago
I have spoken with the owner of this company a few times. Has good attitude. Runs a honest business.
And, the rental is located at the Triana Hostel, Triana Seville. Location is across the river in Seville.
Where many start the Camino from.
By the way, Seville is a very easy city to get around in via bus or even walking.
Only warning I will give you is to be careful of belongings (i.e. wallet, money, cell phone etc) while walking in the tourist areas.
I have never had a problem. But I don't make myself a target for pick pocketing...quick grabs.
Only other suggestion would be, if possible, to have a extra day to see some of the sights in Seville. Or, if enough time go see Granada or Rhonda. Few hours from Seville.
This is good info:
http://www.viaplata.eu
Click the stops on the map for excellent photos and information.