100 milers and posting

[celeste <celestem@badgermining.co.za>]

Here in South Africa we are just beginning to have 100 milers so we still 
have lots to learn.
What I would like to know is what kind of horse (speed wise) is most 
suitable to 100 milers. In my understanding (and I could be completely 
off!) 100 milers are ridden very much slower than 50 milers, so if I had a 
horse thats preferred gait was a slow-medium speed rocking horse canter, 
would this horse be more consistent in 100 milers than for example a horse 
that pulls your arms out of their sockets?
Also can a horse become a 100 miler straight away (assuming offcourse 
he/she has had enough conditioning leading up to the first ride) or do you 
need to work your way up through the lower distances? I ask this because 
there is a theory here that if you plan to do 50's you start on 50's 
otherwise the horse becomes used to 25's and then struggles with the extra 
milage (mentally not physically) when you progress to a 50.
Another question, when I trot on a ride I don't do a full post, I stand in 
my stirrups and just move up and down in the air barely touching the saddle 
as I come down and always leaning slightly forward (hope this paints a 
clear picture!). I have found that this gives my hips less strain during a 
ride (I dislocated my hip a few years back) as all the shock is absorbed 
through my legs. Someone has recently told me however that this puts all my 
weight on just two points on the saddle instead of distributing it evenly 
over tha saddle. Is this correct? I must mention that my horses preferred 
pace is the canter on a ride so I do not do much trotting and I have found 
that when I post this way he strides out much better (longer and faster).

Thanks in advance.
I have already learned so much from all of you!

Celeste (South Africa)
Mawlud (trot...canter.....lets GALLOP!)
Shadixx (trot...canter...gallop...lets WALK or even better BACK!)