I agree with those who have mentioned the importance of footwear and pack weight. They will make HUGE differences. I agree, even more than being "fit enough."
I would check your shoes on at least a couple long walks here and there before you go (I know you don't have the time to dedicate to this regularly, but even one or two long ones will tell you what you need to know re: shoes). Some shoes that feel fine at 5km suddenly are not up to the task at 10km or 20km. Check for blisters, hot or sore spots, and sore knees/hips. And remember that you will be repeating those distances each day on the trail, whereas at home you can rest, heal, adjust, etc, after each walk as needed. Like someone said, better to work out the kinks before you get on the trail.
For shoes, make sure you wear an extra half-size or even full size bigger than you normally wear. Hot weather + many kms = swollen feet!! Add in some downhill terrain where you toes repetitively bump the end of your shoes and you've got a recipe for blisters and lost toenails. Painful and could potentially derail your whole walk.
Re: pack, I would say practice walking with your pack early, and build up to your goal pack weight. Again, what feels totally fine at 5kg can give headaches at 15kg, etc. until you are used to it, and it might take a week to get used to it. Besides that, try to reduce the overall pack weight as much as possible. Again, the first day will feel "lighter" than the 10th day in a row.
Buen Camino!!