Hi, Kitty,
I know that this ascent creates a lot of anxiety, just as the ascent to O Cebreiro does. I think most would agree that the dread was far worse than the reality. I personally think that if you are nervous, Jane's suggestion is a very good one. The scenery is so spectacular and the feeling of one-ness with the universe so strong at points along that walk that it is worth all the exertion. I did what Jane suggests the first time I crossed from St. Jean, stopping in Orisson and spending the night there. I was with my pal Jan Brilleman and his wife, and we were sort of shocked to arrive so early, I think, around noon or so, after leaving St. Jean around 8. (If you're reading this, Jan, is that right? I know you keep detailed notes of these things). We were lucky to have a beautiful day to sit up there and just contemplate the surrounding beauty. And then the walk the next day was extremely relaxed, and not a stress at all. We stopped at the top before the descent, took a long lunch, walked slowly down, and got there after a late start even before the pilgrim's office closed (Jan may correct me). I now know that I can make the crossing in one day without problem, but that time spent up at Orisson was really nice.
For a sense of the elevation gain, take a look at the mundicamino web site. It says there's an 1100 meter gain to the highest point, then 350 meters down to Roncesvalles. (These maps are not precise, but that's probably roughly accurate). If you've done a lot of walking/hiking, you should have a good idea whether that's in the realm of possibility for you. But I really think that for most people, if you're in reasonably good shape, this is a very do-able crossing. Taking it slow and resting helps a lot. And remember that the adrenalin rush of the first day of the Camino is going to keep you going, too!
Laurie