Donald Miller, an author that I respect, wrote a great blog post about the parallels between writing a book and paddling a boat across a large body of water (think really big lake, or a bit of ocean).
To paraphrase: He likens the excited feeling of starting out in the boat to that of starting out on a new story. The early bit goes fast. You feel like you'll be at the end in no time. Then, you get into the middle. The middle is the sticky part where you feel like you're paddling in place and getting nowhere. Yet, the middle is the place that really changes you. (End of paraphrase here...additional note: I'm not doing his writing justice, you should really read the blog, and I was just far too lazy to quote it verbatim).
For years, the primary series was like that for me. The first part of the series was a steady practice early on. It came quickly. Then I spent years paddling in place in Supta Kurmasana feeling like it was never going to change. Only after arriving at the other side and spending a couple years paddling in second series did I start to understand what my teacher meant when he said "Supta Kurmasana is one of the most transformational poses in the primary series." After sharing this with a student who was having his own experiences with Supta Kurmasana I heard my own earlier thoughts spoken back to me. The student said in a skeptical voice "Well, if that's what transformation feels like." And so it does sometimes feel a bit like paddling in place.