I suspect you don't yet need to go clipless, unless there's something really bugging you about plain pedals and sneakers. You get faster and ride farther... but it's a percentage.

That being said, there's absolutely nothing wrong with the mountain bike SPD pedal system. It's doublesided, simple, fairly reliable, fairly lightweight, and inexpensive. The cleat is usually recessed enough that you can go for a walk without falling on your butt. And even fairly low-end pedals will work OK.

OTOH, do not scrimp on shoes. They can and will last you for years before they fall apart. And if you want to ride a century or longer, it's going to be critically important that they be the pinnacle of comfort. I splurged on some fairly fancy sidi shoes and haven't regretted it.