I agree with all the replies. I have only done it once (in this lifetime anyway) and also started from Moissac. I wanted to limber up for a while before hitting the mountains as I didn't have a chance to 'practise' (and practise is NOTHING like the reality) so was pretty limber for an old fart when I got to St. Jean.
Thing is, start where you will but you will meet people who started at St. Jean and if you started in Spain you may feel this sort of niggling feeling that you haven't done it 'properly'. Ludicrous of course - but we are human and you may feel it.
Thing is, not the road, but the cobbled lane in and out of St. Jean is old - no, OLD!. No, really, it is OLD! and you feel it. You feel it deeply. You feel it through your feet, through time, it is old - you belong and it belongs - it is good. When you leave it will be dark as you will be afraid of the ascent and your ability to do it (normal). As you leave that lane through the stone pillars you look back and see - well, you'll have to go there won't you? - and then the hard, a bit frightening if you are afraid of heights sometimes, cold, windy, sunny, all changeable things trek over the pass. During which you will come to a true pass, then a spring where you will stop, then move on and there, in front of you - and GOD (you will think) how mountainous, is Spain - fantastic. And when I stopped at that point on the Camino I looked to my right and there, suspended on an upcurrent, not 20 feet away, was an eagle, just sitting there on the air, gazing at me with one rather cruel looking eye .... I mean ... Crikey! and the long walk along and the descent, where you can see the monastery below you just 4k away and down you go - refuge true and there is even a bar there!! And you feel so tired and so 'Veteran' and you now know that you can do it - really that you can do it, and will do it and life is Wonderful!. If you can, gulp, breathe deep - do not be afraid - and Go from St. Jean!