Your legs will notice it straight away: just after Castrojeriz the road climbs, short and steep, to the Alto de Mostelares. At the top you’re at just over 900 metres. After that the Meseta opens up in front of you, and you can clear your head with every step you take.
The descent is long and broad, followed by straight stretches with little shade. At the Puente de Itero you cross the Pisuerga and walk into Palencia. Around 7 hours of walking is very doable if you keep your pace steady. The last stretch follows the canal to Frómista, where the Romanesque church of San Martín awaits you. Take the climb at your own pace, and the rest will take care of itself.