I did the exact same thing last year. I've never done bronze either, therefore I had to do 6 months of volunteering, 6 months skill and 3 months physical. However if you want to do 6 months physical, 6 months volunteering and 3 months skill that's fine, as long as you did something for 3 months and whatever is left for 6 months then you're okay. Skills and physical activities can be anything, I did photography as a skill and physical, well I just went to the gym. You can look up the list of what you can do online or just ask your supervisor ? I volunteered in British Heart Foundation and it was quite fun, 6 months went by really fast.
The expedition itself is.. well hard, so be ready for that in advance, however if that helps the experience is fun. Expedition lasted for 3 days if I remember it right and each day we had to walk around 6-7 hours so be ready for back ache, feet ache and exhaustion as you will be sleeping in a tent. On a good side its only 3 days you can deal with it, it will be totally worth it.