G'Day,
@Paul_L Another welcome to the forum. I cycled and walked the
Camino Frances - cycled 2015 (from Pamplona) and walked 2017 from St Jean.
You do not mention (I think) which route or camino you are intending to travel. So yes you do, imho, need hill training. There are more than a few hills in Spain. The regular guide book for the
Camino Frances (by
Brierley) is good for planning but not all that good for cyclists. There is another book, sponsored by Michellin (the tyre people), which provides details of alternative roads available when the walking track is unsuitable. I will try to do a search of other guide books that help.
Are you bringing your own bike or going to hire one?? If you are hirering then I strongly recommend bringing your own locks and chains. If you have more questions feel free to ask. Buen Camino