The terrain makes the biking challenging regardless of the distance. There are some steep mountain ascents and descents, as well as some short steep climbs on the last day. You don’t have to be an expert mountain biker but you should be comfortable on loose gravel and some rocky terrain.
There is a combination of gravel and asphalt. For the French route often times there is a paved road option and a gravel option. You could ride 50% gravel and 50% road; or you could ride 90% road and 10% gravel. Most groups fall somewhere in the middle of those two extremes. (At times the gravel path is right next to the road so it doesn’t make much sense to bother the walkers; hence the 50% road.)
The Portuguese route doesn’t allow for the different options. It is a mix of road, gravel, and rocky paths.
There are some busier roads as well as traffic in towns.
If you get tired or sick and want to take a day off, you can take taxis between towns.
The bikes come with panniers to carry whatever you need for the day.