I would walk 500 miles... Or would I?

I walk and hike a lot for exercise and to meet my dog's needs. But I'm having trouble going longer distances without some hip pain and tiredness. I just signed up for a 12 mile hike with 2200+ feet elevation gain in early December with friends. I don't want to have to back out. How can I up my mileage? I currently walk or hike every day:
- At least 5-7 miles one day a week
- 4+ miles another two days a week
- Anywhere from 1-3 miles the other days of the week
- It's pretty much my only exercise
- A few of the walks have hills (1300 ft elevation gain on the high end), the short ones are flat
- I'm 41, female, and on the high end of normal weight
- While I get winded on hills, getting winded is not my problem with longer distances, it's fatigue and hip pain
- I generally walk between 3.5 and 4 miles an hour

I did a 9+ mile hike with about 600+ feet elevation change yesterday and could not have walked another step at the end. My hips were killing me the last mile and I was worn out. How between now and the first week of December can I get to 12 miles and steep inclines without hurting myself? I have lots of time to practice between now and then, but I'm not sure what would be the best approach to do that.