Assuming the carpet does not have an anti-grime enchantment on the underside (which you would wonder why it'd have one if it was meant for a Manehattan chateau), the unprotected side gains its first layer of dust and slight moistness. Physics' fire spell had scorched the top of the cavern and chipped a bit of the looser stalactites, causing a small rain of dust, ash and gravel that settled onto clothing, fur and accessories that weren't enchanted or otherwise protected. Due to the strain of the relatively long twenty-mile journey, all of them also have quite a bit of dried sweat and dust from the trampled road on them, and some (such as Fatal) had bits of dried blood staining their fur or clothing as well, either their own or that of another pony (as in the case of Physics). Some had scratches and cuts that they never even noticed till now on their hooves and legs, whether from sharp rocks or the occasional thorny branch on the road.
The mosquitoes were beginning to find their marks now, unless somepony had spray or enchantments. Soon most would have uncomfortable insect bites in addition to the other discomforts of the wilderness.
As Celestia's sun began to set beyond the forest to the immediate west, behind where they assumed the Eastern Gate still stood, a sudden chill and darkness began to grow, unexpectedly quickly for those unaccustomed to watching the patterns of untamed weather. Tonight's chill would not be icy, but cold enough to induce some form of pneumonia if a fire wasn't built.
Physics, upon observation of the forested landscape, finds himself slightly relieved to see an abundance of grass and wild mustard growing here and there, along with some thistles and the occasional berry or nut. It might not be gourmet food, but the ponies, at least, would not starve for a while in this climate. There were also a few mushrooms growing on rotten logs here and there, which might be useful for medicine, food or poison as necessary.
Amazingly, if anypony cared to take a closer look, there seemed to be quite a few wild mint plants also, and most likely there would be cattail and stinging nettle near any sources of water, quite nutritious and filled with protein if boiled.