Hola
@snowgorunner , another welcome to the Forum and to Camino life.
Martysevelle has covered just about all the point I recommend. I cannot stress too highly how hot it will be on the Meseta (Burgos to Leon). As you are on the bike you will be travelling faster but will also consume more energy and thus will need more water - as much as 4/5 litres per day.
As for getting a suitable bike - I totally agree about buying v renting. Either way either bring or buy a very, very good lock & bike chain and always secure your bike when you are away from it. Most albergues have rules about when they will admit bike riders (the donativo ones often make you wait until 4.00 PM or 16.00; the private ones are not so fussy). The better places will provide a place for you to secure your bike overnight (but again make sure you chain it). With the hot weather you will want to be on the road early, but as you get further west (especially after Astorga) the sun rise is later, anything up to 30 mins later between Pamplona/Barcelona and Astorga so you made be riding in the dark or early twilight and even with a light on your bike I would advise waiting until just on sunrise.
Route to follow: there are places on the
Camino Frances where it is totally unsuitable for even advanced mountain bikers, so be prepared to ride the roads. Don't worry too much Spanish drivers are used to cyclist and are usually more respectful. One section I strongly recommend following the road is from Rabanal to Molinaseca (the section up to the Cruz de Ferro), I have both walked this section and cycled it - not suitable for bikes of any description. Good luck; Buen Camino.