How Fast Can I Go?
Many variables will affect your average daily distance covered... your fitness, the weather, the weight your carrying, your technical level, your determination, your patience, whether you stay on the trail or alternate with the roads, the amount and length of your stops, difficulty of that particular section, etc.
I'll try and give some basic guidelines as to how to estimate the distance you can cover. There are however a few things you must bear in mind.
The objective here is to estimate the average daily distance in order to calculate the total distance you can cover in the time you have. This is the ONLY use for the average daily distance. When actually riding the difference between the longest and shortest days will be great and can exceed 500%.
Doing the Camino will require an effort; more so than any of your previous riding experiences (unless you routinely do 6 to 7 day bike trips in which case you should probably just go see the pictures). No matter what your fitness level you will tax it during the trip. You'll be carrying at least 8Kg more than on your usual rides, you'll be riding longer and fatigue accumulates when you ride everyday for more than 3 or 4 days at a time.
All but the fittest and most skilled bikers will have to get off and push the bike at one point or another. It might total no more than a few hundred yards for a 900km trip or it might be quite a bit more than that. On difficult surfaces a couple of yards walking can save quite a bit of energy, relieve the butt and even spare you a trip-ending fall.
Determine your fitness level
Based on this very empirical classification find the description that most closely describes you in terms of your activity or what you'll sustain in the 12 weeks (at least) prior to the trip.
The couch potato. Last rode a bike when he could still see his feet... go no further; do not attempt to ride the Camino.
The weekend warrior. Rides once a week on the road. Covers 30Km to 40Km on each ride and takes from 2:30 to 3:30 hours. No real hills.
The average biker. Rides two or three times a week. One long ride on the weekends covering up to 60Km on the road or 40Km on trails. Some rides include long climbs (at least 45 min constant accent on inclines of 6% or more.
The top 5%. Rides 3 to 5 times a week. One or two long rides covering up to 60Km on trails and totaling 180Km to 250Km a week. Avoids roads when possible. Anybody at this level or above can skip this section. You won't need any help determining how much you can do.
Few will fit any of the above exactly but find a "zone" between the descriptions and adjust recommendations accordingly.
Calculating the distance
Suggested distances are based on good weather (no significant rain or wind), a 25Kg load including the bike, and 100% trail riding. Adjustment factors are given for ranges in these variables.
The weekend warrior can probably average 35Km to 45Km a day. The upper limit will be pretty hard; the lower still no piece of cake.
80Km a day will probably be the very upper limit for the average biker. If you want to do anything other than ride, eat and sleep, I suggest aiming a little lower.
Pouring rain on the flat sections will probably slow you down no more than 10% - 15%. On the hilly sections you might not be able to cover half the planned distance for a day. Some sections may become down right impassable.
Going on the road will cut your effort by 30% to 50%.. If you did the Camino only on the road you could almost double your average distance with equal effort when compared to staying always on the trail.
Wind is very unpredictable. It can virtually slow you to a crawl though this is seldom the case.
I have no measurable estimate on how weight affects effort but EVERY GRAM COUNTS!