I have walked Santiago to SJPdP three times now, including once in winter.
The
Camino Frances is only marked to Santiago, not back again (unlike pilgrim routes in other countries that are marked both ways). There have been some unofficial attempts to marking a reverse route, there are a few blue spiral arrow markings but you cannot count on them.
However, walking back is not so difficult as you may think. Most times on the Frances if in doubt wait for a pilgrim heading towards you, this won't take long most months (not winter) and not late afternoons when no-one is walking.
Carry an app with the route marked on it (I use mapsme. with gpx files for instance) these really help.
Be prepared for locals to stop you more than once per day telling you you are going the wrong way!
The walk in my opinion in harder terrain wise - Out of Molinesca towards Cruz de Ferro is a killer, as is Alto de Perdon in reverse. However, I find the pyrenees crossings easier, Napoleon route but especially the Valcarlos route.
On sunny days the sun is in your eyes most of the morning, I needed sunglasse on the meseta. .
It is lonely, EVERY pilgrim will pass you for one second (that is hundreds of Buen Camino's a day)! But everyone will know everyone else in the albergues at night, you will know no-one all the way (unless you meet someone else walking back too, which I have done a few times). You will meet people in the evenings at the albergues who you really get on with knowing you will never see them again.
Other pilgrims may think you are either a weirdo, insane or a even a hero. Some will totally ignore you. One thought I was cheating! However hospitalero's have seen it all before and will understand you and many will give you a bit of kudos too. No problems staying in albergues, just use your credential as before.
So, if you know these things in advance it is very do-able. I enjoy it immensely, the pressure is off, the Way looks and feels totally different. Go for it!
Buen Camino
Davey