We did over a week a while back, but got the inlaws (who live by the Cotswold Way) to come collect us every evening so we could abuse their hospitality. As said, walking isn't cycling, different muscles and all that – 17 miles is a fair distance, probably six solid hours afoot plus time for breaks, food, etc.
We hike a lot and have been doing so for years (seventeen or so miles is a lunchtime start and finish at beer o'clock). Decent footwear is a given. I always wear boots for the ankle support but make sure you're used to them. As with cycling, that minor niggle at the start will be painful by the end of any walk. Multiply that by several days and you'll be confessing secrets to anyone that asks. Make sure you have a good combo and boots and socks, blisters are a curse. Moisturizer or similar applied to the foot before pulling the sock on is a good idea and helps reduce friction.
Take a torch, it's easy to get caught out by sundown, and it's quickly too dark to find your footing. Also, country lanes, as any cyclist knows, are the place for idiot driving, more so after dark and they've no more respect for walkers, so you need illumination.
Paper OS maps, they are a lot easier than squinting at a phone. Some supplies, but the Cotswold Way is rarely far from a gastropub so you'll probably not die and bear sightings are rare. You can probably get an Uber.
But yes, don't assume 17 miles/day is easy. It's what we did on Saturday across the South Downs and we were pretty knackered by the end (admittedly it was hilly and windy).