Hi,
Collecting petunia seed is easy. The seed pods are small and teardrop shaped. They turn light brown and get crisp when ready to harvest the seed. The seed is very small and dark.
I'm not sure about distance, but I'm going to assume it is fairly far, and if you wish to save seeds should only grow one variety. Remember though that many varieties of petunias sold are hybrids and will not come true from seed. So only grow an open pollinated variety if you wish to save seed.
Remy

I can tell you that a neigbhor, an old woman, has the same petunia patch of mixed colors year fter year. You'd think the colors would all become the same after so many years, but she still has pink, pure white, and lavender. I think thiws is a very old-fashioned type, though. When they paved are orad and thus stirred up old soil, that type of old petunia sprouted up (over a mile from the old woman). There were a few colors there, too. I mananged to kill the seed and cuttings I saved. Dang it. The white ones, though, even overwintered a few years in a protected spot agianst the concrete foundation of the house.