How long the pony can drive is dependent on a number of things, the first being long, slow conditioning. Terrain, is it flat or hilly. method of hitching can be critical. and of course, speed on course. Acknowledge that you can pull your pony any time you feel it's gone far enough. Don't let anyone pressure you into going farther than the pony can handle.
Cart balance will be critical, as well as the width of your harness saddle. Carrying weight is very fatiguing for a horse that is also pulling it. On a light cart, small shifts of your body weight can add or subtract many pounds across the saddle, so you need to actively balance the cart for the pony's optimal comfort and use a saddle that's wide with adequate spine clearance..
The breast collar you select must also be wide enough to distribute weight across the chest. If your equipment is dialed in, your cart balance is optimal and you take plenty of time to condition, your pony should be able to go the distance.
I would also research how endurance riders prepare their horses, care during a ride, and critically, post ride care. I once did a distance drive with a pair of 11.2 hand ponies and my mare beat all the horses for best conditioned.