Karen, for many the first day will indeed be one of the most physically arduous of the
Camino Frances. It certainly was for me, as someone who walked the Napolean route to Roncesvalles with limited training.
I found the stretch from St Jean to Orisson particularly demanding, and had to stop every now and then to take a breather before continuing. But continue I did. Once I reached Orisson and continued with others, I found the rest of the hike to Lepoeder much more manageable, dare I say even enjoyable. The scenery along the way is spectacular and you will have plenty of company, be they fellow pilgrims, horses, and goats.
As the saying goes, 'what goes up must come down'. The view from the top looking down at Roncesvalles and Spain lying below in the valley was a particular favourite of mine. I wished that descent walk through the woods could have lasted forever.
I think I enjoyed it so much because I was worried beforehand about my ability to hike uphill stages. I hated the thought of walking uphill, actually still do, but overcoming that obstacle (a mental one as much as physical) was liberating for me and a great confidence boost for every day's walking after that.
Without knowing your preparation or fitness levels, I would say to 'go for it'. Leave early and take your time, take rests when required, use those rest times to chat to others. Walking with others can be easier on days like the first one when we are naturally less confident of our esteem levels. Try and get as much uphill walking as you can get before now and the time you go. Every bit will help. If you have an extra day in St Jean, walk the first hill outside town a few times. I did, up and down five times, and it helped to some small degree the following morning.
By way of reference, I left St Jean at 6.30am and arrived at Orisson just after 8.45am. Reached the highest point at 1pm, and after taking our time to have a picnic in the woods, reached Roncesvalles before 3.00pm. Eight to eight and a half hours duration in total, we could have done it in seven and a half if pushed.
That first day was my favourite stage by far, for many reasons. It was a bit of an emotional roller-coaster, but so too is the Camino. I still struggled on the subsequent uphill stages (Castrojeriz, O Ceibeiro) but nowhere near to the same extent. By then I had 'found my legs' and there was no stopping me. I never felt the same euphoria of making it to the top like I did that day on the Pyrennes. I had to wait until arriving into Santiago to re-discover the same degree of satisfaction.
Take note of the weather conditions before you decide anything. We walked in August when the weather wasn't really a factor. Good luck in whatever decision you make. The first few days of the Camino can be daunting in general. The scale of it all seems almost too much to comprehend. Break it down instead into short, realistic goals. First to Orisson, then Roncesvalles, re-access over a cerveza and off you go again. Before you know it, it will all seem to be going by too quick and you will want to slow everything down again. Buen camino.