Hey CarylAnn, I envy you biking the CF. I debated trying that a few years back instead of hiking. I chose hiking at the time. I read the forum daily, but comment rarely. I am a recreational biker, both road an trail, so I thought I might impart some of my "wisdom" based on similar experiences.
I agree with the above regarding padded shorts. I bike frequently, but undertook a ride near the distance you will undertake only twice on rail trails (relatively flat) in the U.S.
Several years ago a group of us biked the GAP (Great Allegheny Passage) and C&O Canal. A few years later I biked the KATY in Missouri. I was in my 60s for both. Both were on the order of 300 miles over the course of 8 days. You will definitely regret not having padding as you admit to not being an experienced distance rider. I also agree with a crotch lubricant to reduce friction. Even using the cream, I still got sore by the end of the rides. Beyond that, be prepared with clothing options. I have separate leggings and sleeves that might be worn on chilly mornings and removed as the day warms. The reverse is true when approaching sizable hills/mountains. You will heat up as you work up a hill and may chill coasting down a hill. A windbreaker shell might also be useful for cool days. Don't forget the wind chill effect on a bike. I would also suggest padded biking gloves. I use gloves every time I ride. I have finger-less and fingered gloves. I become overheated and sweat easily. I also don't like to be cold, so I look for ways to help control my body temp.
You don't mention if you are doing this on your own or as part of a supported group tour. I have a "trunk" that mounts to a "book rack" on the back of my bike. This carries the excess equipment/clothing. Panniers could also be used. When I did the GAP/C&O, we did that with an outfitter who transported most of our stuff from destination to destination and provided lunches, so I used only the trunk. When I rode the KATY, I was on my own and used the trunk
and panniers to carry everything.
I'd suggest full zip jerseys to be adjusted for cooling (I bought a souvenir full-zip shirt in Santiago after hiking the CF a few years ago).
If you want the best clothing for this ride, I recommend you investigate cycling clothing. It's not inexpensive, and you usually get what you pay for. If you choose to buy this, buy it in advance and use it. Check for comfort, especially the shorts. As mentioned by Tandem Graham, seams in the wrong place, or just the wrong fit will cause suffering over a ride of this distance.
One other request. As you approach hikers from behind, give ample, loud notice that you are approaching. I was appalled at how few bikers gave warning on the CF. Frequently, bikers would fly by with no notice. I was startled several times as I did not hear them approaching, and I'm pretty alert to changes in my environment. This is even more important now as many walkers wear ear buds that drown out noise. I use a bell on my trail rides and many times walkers don't hear me approaching for that reason. I have to shout to get their attention.
One last recommendation -- WEAR A HELMET! I don't get on 2 wheels without a helmet. I've been riding motorcycles for 50 years and bicycles longer. I simply don't understand why some people refuse to wear helmets, other than it being a macho thing. I've had near misses and accidents that were lessened by the fact that I wear a helmet. I have bike helmets with scars on them instead of on my head. Accidents can happen anytime - sometimes when you least expect it.
I hope you enjoy your ride. I still think about my Camino every day and long for another experience. I made several friends that I still have occasional contact with. I'm considering doing the Portuguese from Porto while I'm still able (I think!), but have not made any plans. Maybe in 2021 for my 70th birthday????
Happy riding!!!