Hi Angela - Bordeaux to St Jean starts of with days of being flat and level. This is the area named Les Landes and was once under the sea. It is now all pine forests but there are almost no tracks so there is a few days walking with traffic and absolutely nothing to see except pine tress - there are also some large horse flies down there too.
Possibly a more pleasant route would be to pick up on the Le Puy Camino route as it is rather beautiful and goes through some lovely towns and villages. Moissac is a good starting place as it has a main line train station so is easy to get to - have a look at the route maps you can find online. Another plus for starting somewhere like Moissac is that you would eventually go through the town of Condom - the perfect place to send postcards home telling people you are being safe
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Time to Santiago? Who knows? Guide books go for 35 days-ish from St Jean but this does not allow for injury or rest days .. if you want to start somewhere in France and also take it easy you might be better factoring in at least 50 days.
Signage in France is not quite like Spain - the routes go along French walking routes - the GR - and they have their own colours. As they are for walkers as well as pilgrims they tend to be marked pretty well.
I can't answer the pre-reservation question as I never do that, I never know how far I will walk and am quite happy to see how it turns out at the end of the day, each day.
Spring in western Europe is normally pleasant, warm and sunny - but we have been having odd weather systems the last few years- it could be hot and sunny, it could be cold rain, it could even snow! so you would need to keep an eye on the forecasts as you get closer to going - but on the whole it is usually a lovely time to walk. mind you, I was out doing first aid in April this year and it was freezing!! So cold that I became ill. (Or maybe me being ill made me think it was colder than it was).
The danger you speak of I also cannot answer. Bad things happen everywhere, true, but on the whole Caminos are safe. Though there will be less pilgrims in France so you will be walking alone a lot of the time. I have never heard of a pilgrim having problems in France.
All the above is not 'truth' but only my opinion, so do make your own choices after getting in all the info.
Enjoy the planning!!