I work on a horse can pull one and a half times it's body weight comfortably

 

http://www.horseforum.com/new-horses/how-strong-horses-really-569690/
This talks about how much they can pull. Really it depends on the size of your team, how well they are conditioned, the route you are traveling, the weather conditions (hot, humid vs cool, dry), road conditions, incline, type of vehicle and how smooth it travels as well as your harness and rigging and the amount of weight actually on your animals.Your actual load weight does not equal the amount of draft. How many passengers and are you carrying anything else?

 

Size of team would be 14-14.2hh, two horses (well ponies technically).

Type of vehicle is a wagonette. Reasonably large wheels, with every possible effort to ensure it travels smoothly, lightly and with as little friction as possible. Road is a normal tarmac road. Route includes several hills, but nothing horribly steep.

The harness has not yet been bought, and will be chosen to fit the load and conditions.

So the discrepancy of those figures is due to the actual amount of draft, not the physical weight of the load? Eg the 10% is relating to a heavy drag on the ground, whereas the "carriage horses taking 2-3x body weight" is relating to a very well built and packed carriage probably with ball bearings, on ideal flat ground?

Load theoretically includes two people, one average size and one small, and yes includes other stuff as well. May include minor luggage, food, camping gear (small tent), small bits of horse gear, plus feed for the horses for two or three days, (Bearing in mind that grazing stops are available as well, so they don't have to carry 100% of their diet) including some water to provide them if we can't find a good stream every stop. Lacking a reliable car, it is likely that sometimes we'd need to combine the visiting and camping pleasure jaunt with bringing the groceries home on the return trip. Of course much of the feed will be gone by then to counter that.

I know horses used to do this sort of thing all the time, but these are not draft animals so I don't know if it may be outside their capabilities.

 

I have found this REALLY depends on the work ethic of the horse. I have a Morgan who pulls wagon loads of watermelons out of the field single. He weighs the same 950 lbs as most of my horse.
Several of my other horses do not like pulling 2 people on the cart where normally I drive by myself. They have the strength, but not the " want to".

I don't think you will have much trouble, as long as they have the stamina to how that long.

 

I'm not very experienced at all, but I can shed some light on the range of numbers you got.

Horses can generally pull 10% of their weight, in draft, over long distances. 15% of their weight is considered the maximum for long distances.

Weight wise, horses can pull 200% of their weight. The difference being that a horse doesn't have to exert a pound of effort to pull a pound.

Tillers international had a very nice paper explaining this, though it was aimed towards cattle.
(It's too big to post here, but I included some examples below)
4,000lb wagon on gravel road with pneumatic tires:169'lbf'
lbf= pounds per foot. I've not been able to decipher whether this refers to width or length of the wagon.
I also have numbers for plowing, logging, sleds/stoneboats and harrowing, if you're interested in using horses for traction.

 

Some great comments were given, enjoyed reading. You may want to also watch for how hard they sweat, are they breathing hard, or heavy, do they hesitate to go up hills or down, suddenly stop while driving, all things to know. Big thing is know your team, start them out driving and get them into condition before asking more of them. If your going over long periods of driving during the day, let them walk, do some trotting to break up the day.

Are they already broke to drive?

 

Oh yes I will definately watch their behaviour sweating etc to see how they're doing. A mostly quiet, easy pace will be the order of the day, and rest stops as needed too.

They're not yet broken to drive, we're starting on that now. We're also starting on getting the cart and harness, so I'm finding out now what sort of weight and therefore size cart will be appropriate.

We'll be taking our time through the training process, and giving a slow increase in work to condition them properly before asking for any long trips.

 

You may want to look around for a horse size fore cart. They are two wheels so not as easy to get yourself into trouble with a young team, a bit heavier so they will have to work and build muscle to pull it and fun to drive. Then you can work your way up from there....

Please, photo's are a must and be safe. Look forward to hearing more of you adventures.