Walked the Ingles earlier this year. 5 days, all days over 20km except the last one, and the longest stage 32km. The Ingles also is not flat.
Am in my 50's, do no sport and did no training. But, living in London, on most days I walk 10km, over the whole day, and regularly walk 20 to 30 minutes faster than the traffic to the shops, etc. Reasoned that walking 30km at even a slow pace of 4 km per hour is 8 hours slow walk, 9 hours with lunch, or 10 hours with lunch and rests. Therefore, doable, as the main purpose each day is to walk to your destination. Once I had done that, the fear of walking 20km, 25km, or 30km distances per day became much less. Just put one foot in front of me, counted down the km on the waymarks as to where I was staying each night, took rests where I could / needed (albeit in the week I walked, that meant taking shelter from the rain) and just took it slow and steady.
The key thing, as many have posted above is to make sure you are comfortable in whatever footwear you will walk in, and if the footwear is new, break them in. Also the advice from
@Cybermum to walk some time with your pack on is also invaluable. If walking with a pack doesn't feel comfortable, consider sending the packs onto your destination by Correos each day. There is no cheating or shame in doing this, especially if it means that you can effectively take your creature comforts with you.
What kept me going was walking with plenty of bottled water, fruit, snacks and chocolate. I also stayed in pensions and casa rurals and giving my legs a good soak in a hot bath where I could did wonders for keeping them going for the next day.
If you are walking for 5 days, consider the Ingles from Ferrol. You walk a whole Camino start to finish (which, believe me, does give you a sense of achievement when you complete it), you arrive in Santiago, qualify for a compostela, the first two days are picturesque walking round the coast, and then, when I have the ability to be able to have longer times off, sets you up for the urge to walk longer Caminos.