I strongly disagree with some of those guidebooks (guidebooks are evil), that the first day up to Roncesvalles is the hardest on the Camino Frances. That honor belongs to O Cebreiro, which truly rewards the pilgrim for their efforts with amazing views, a visit to Don Elias’ grave, and good food on top, but still takes the most out of them.
That isn’t the whole ordeal, though. Anyone deciding to walk further will, mere kilometers after, receive the Alto do Poio’s “steepest incline of the Camino” achievement. After that, though, all the hard parts of the Camino are dealt with.