If your goal is to become a faster rider, then you should ride with fast riders, especially ones who are willing to teach. If you've found such a group, stick with them. Sure, you'll get dropped (as we all have), but if you truly try your best to hang on for as long as you can, that level of effort will pay off big over time. You'll see far more improvement than riding slower over a longer distance.

If, on the other hand, speed is not your thing, but you'd rather do more touring, then consider DC Rand: http://dcrand.org/dcr/ I know a number of folks in that group; you'd be hard pressed to find more dedicated, knowledgeable and friendly folks.