Hello Will:
Most of the people take big backpacks but half full, few people manage to fit everything in a day-pack.
Some tips to fit everything in a day-pack:
1) Bring a small sleeping bag that you can tie to your day-pack.
2) Bring some straps to tie things to your day-pack as your sleeping bag and sleeping pad or foam-mattress.
3) Be awared that even your tour provides a foam-pad, you have to carry it.
4) You have to carry a personal bottle of water, so I suggest to buy at the beginning of the trail a cheap bottle carrier.
5) Your camera will have to go outside of your day-pack, something else around your neck.
Certainly you can fit everything in your day-pack, but you will end up with other things hanging from your neck and shoulders.
In regard to clothing, you can make it with one pair of trousers, one shorts, 2 t-shirts, a polar fleece, a wind-breaker and enough underwear and socks for 4 days. I suggest to bring also sandals, gloves and a woolly-hut. Beside clothing you will need a small towel, personal items, sun-block and bug repellent.
Have a great trip,
Ivan
[ Edit: Sorry, no promos please... ]