Like I say for rock climbing, the same thing can be said for backpacking, there is being in shape and then being in shape for rock climbing or in your case being in shape for backpacking, since you`ll be using muscles you didnt know you had. No one on here can tell you what your trail speed will be. There is no possible way some one over the internet can do that for you without knowing you.
There are so many variables from your pack weight to elevation gain to how trail ready you really are. I`d say your best bet is to do some weekend trip either on the trail you are going on or one with similar or harder terrain and then extrapolate from your experience from the shorter weekend trips. Consider it a shakedown hike. Bring the gear you will be using, this way you can tweak it before you are ion the long haul. At the end of the weekend go through your pack and pull out the things you didnt touch. It is so easy to over pack, especially with clothes that you will never wear.
Feel free to e-mail me if you have any questions