Irulan's advice -- "direct, clear and tactful" -- is perfect. If you want to, make it all about you -- say you need to focus on training, and you realized you can't do that when riding with other people. But definitely don't feel obligated to continue cycling with these two. It won't get better.

Their behavior sounds extremely self-centered. I'm a very slow rider -- there's nothing wrong with that. But I'm well aware that my pace would not be right for many (maybe most) cyclists.