We all have our own agendas, our own needs and desires and it's not for any of us to tell another what to do. However, seeing you asked ...
I, too, have done a few long distance walks and also a few pilgrimages. They are different things. Yes, they both involve walking every day and often involve sleeping in dormitory accommodation but I've always found a difference in the people I've met along the way and a difference in the way I walk. The pilgrim routes favour reflection and some days that means walking at a pace that is slower than your usual. There is no hurry and it's wonderful if you can allow it to unfold on its own.
For a 24 year old with experience of long distance walking, I wouldn't think that St Jean to Roncesvalles would present you with any great difficulties providing that, as Laurie pointed out, you were walking in ideal conditions. I've walked that stage twice, once at 54 and once at 56. A few years on and I wouldn't hesitate to walk it again.
However, you are likely to be quite tired and would be setting out late which would mean that your first day would be a race. Do you really want to start your Camino like that? St Jean is such a lovely town, why not enjoy the ambiance and meet up with a few other pilgrims?