@Barbara Whelan , welcome to the forum.
I cant help you with your question as to when to start.
But I can ask you a question. Do you think you have sufficiently prepared to walk between 20 and 30 km per day for between 30 and 45 days (assuming you have a lay day every 6 days or so)?
And its not just that but also the ups and downs (often the downs can be no less difficult than the ups). This is not meant to worry you. My only wish is that you fully succeed in achieving your intention without any signficant issues.
I was somewhat lucky in having nearly four years to prepare before my first day in April 2016. A test I have developed for myself for my continued training is to continue achieving:
1) 15 km before stopping for breakfast; and
2) 700 metres of elevation gain before ...
The second is hard for me at present - the route is crossed by the construction of a motorway until April 2020. Some people who live in low lying areas say they walk up stairs in high rise buildings - even better in some respects.
The purpose of these, for me at least, is to not only maintain (build) my bodily stamina but also help my mind when things dont turn out as intended.
Another aspect is to critically review and seriously use all clothing and other gear. I have just finished 600 km from Canterbury Cathedral to Chaumont on the upper Marne (less than 200 km from Switzerland), my fifth long distance pilgrimage in three years. When I got home last week I still managed to discard several items (a third set of socks and hose in particular) for future trips.
You've made a good choice, in my view, not to book beyond the first two days. Nice to be free to let your pilgrimage evolve as you complete each day. I cant recall any serious issues getting accomodation each day over my five trips in the past three years.
Kia kaha (take care, be strong, get going)