One week isn't long to prepare, but if you're reasonably fit and have decent kit, particularly comfortable footwear, you should be fine. I did a 40 mile walk, Keswick to Barrow in Lake District, north west England, in May, from a basis of being a fell walker, orienteer, runner, cyclist etc. Although there were food and drink support points, I carried some food, so that I would never be stuck if I needed to eat.
I suggest you check the information supplied to walkers, so you know what to bring, what is supplied, timings etc. If you haven' walked far in the shoes / boots you intend to wear, then I'd go out and walk at least 5km at one time, to see if there are any sore points on your feet. If you feel any soreness, immediately adjust socks - sometimes seems rub on toes etc - and / or add / remove a pair of socks / and adjust lacing. If needed, apply plasters sooner rather than later - when skin is red, but before it breaks. Remember that feet usually swell through the day, so it's normal to loosen laces slightly in a day's walk. Also check your rucksack / bumbag is comforatble, adjusted correctly etc.
Any time outside in these islands in summer can mean anything from bright sunshine to heavy rain, so I'd never go out for 28km without waterproof jacket and probably trousers and possibly gaiters [depending on surface you're walking on], plus sunscreen, hat, gloves, spare top etc.
Enjoy your walk!