Hola and welcome to the forum
Your plan to walk Norte or Frances in 35 days should be fine as many pilgrims in different ages and physical conditions do that every month of the year.
The Frances has more pilgrims than Norte, but in you will not be alone if you choose Norte.
There will probably be nights where you will be saying alone at an albergue, but this I found a pleasant opportunity to read and relax.
Some albergues (municipal and xunta - the cheaper ones) on Norte will be closed, but there will be private albergues as well as hostels/hotels and youth hostels open, at a little bit higher cost.
Regarding food, I would recommend that you always carry something in case you will arrive at a smaller town without a shop/tienda.
Powder soup, bread, cheese, chorizo or something light to carry.
Also, whenever you pass some shopping opportunity, do make use of it, as you will not know how much longer it is until the next opportunity.
Trail conditions on Norte are good and generally well marked.
First week until Bilbao can be a challenge as much is through uneven ground and forests which are stunning.
Do remember that one of the differences between walking in summer compared with autumn/winter, is the shorter and diminishing daylight.
When you are to walk a longer distance day, you may have to begin before or at daylight and will end up late.
A good lamp will be essential.
The tourist information in Henday or Irun will issue a credential. You may want to write or call them before your departure to make sure.
Have a look at these threads:
https://www.caminodesantiago.me/community/threads/irun-place-to-stay-october-1st.29354/#post-246877
https://www.caminodesantiago.me/community/threads/accommodation-irun-day-one.27622/#post-240274
https://www.caminodesantiago.me/community/threads/camino-del-norte-2014.25220/#post-208745
Whether to walk Norte or Frances, it is really up to you.
They are both great walks and to follow you gut feeling may be the right thing to do.
Personally I walked Frances first and normally I recommend this, as it is a bit 'easier' (more albergues open, more pilgrims), but many pilgrims who walked Norte as their first, have stated that they had a fantastic experience.
Last thing you may want to read up a little on these last days before your departure, is Camino Primitivo.
You will have the opportunity to continue along the coast on Norte or walk Primitivo through the mountains once you get to the fork at Villaviciosa.
You don't have to decide now, but having an idea about both routes gives you some opportunities.
Camino Primitivo is a great walk, but it will also depend on what weather you will get.
I hope this helps and have a great walk.
P.S
Here is a link about 'off season' walking on Norte.
https://www.caminodesantiago.me/community/threads/norte-in-april-advice-and-stories-welcome.24307/
Buen Camino
Lettinggo