From someone who likes to cover at least 40 mile a day on multi day routes, I think you will struggle to maintain that day in day out, being a long distance novice. It is particularly hard on the feet and legs if keeping to roads. you will definately need good supportive well cushioned walking shoes or trainers. blisters will be a reality so go online and search for foot taping methods and get some good tape. Remove any hard skin from your feet especially at the heels, side of outer toes and tor tips, cut toe nails. Try to keep the feet dry. Set your goal as you have - London, but just aim to get as far as you can so that if you have to end your challenge earlier, you are not too dissapointed. It's about having a go, the journey is as much, if not more, than the destination. Good luck.