My experience was similar to David's--I didn't find the route as hard as I had feared. I started from SJdPP the morning of Sept. 6, a beautiful day, and somewhere along the route met a friendly young British woman as she, I, and a couple of guys had stopped to catch our breath. She and I continued the walk together and were able to talk as we went, which is always a good sign.
When we got to Orisson (I stayed overnight), we were tired but not unreasonably so. However, as I was talking to a young woman who worked there, I mentioned my fears about the next day's walk which I assumed would be much harder. I could see how the road led up, and up, and up .... She told me I had just completed the most difficult part and that the next day's ascent would be easier.
Now the walk
down to Roncesvalles was hard, most particularly in the beautiful woods. (I took the path, not the road.) The only time I fell during any of my camino was in a steep stretch where it seemed that despite assistance from my poles, my knees just gave way, causing me to sit down hard and then tip back b/c of my pack. Luckily, three German girls were walking by me and helped me up from my turtle-like position! No injury except to my dignity.
By the time I reached Roncesvalles, I was truly tired but uninjured, I had accomplished what for my 68-year-old self was an amazing feat, and I had seen glorious views. I'd do it again.