herche wrote: I was curious what kind of 'curve' of improvement to expect.
Certainly you're right that the first year leaps and bounds are the biggest fastest gains, numerically. But IME the relatively slower gains later are more than compensated for by being more meaningful - you know how hard you worked for that last twenty seconds of improvement, it therefore becomes inherently more meaningful.
And real life seldom follows a precise curve, let alone everyone's curves being the same shape. Me, I started in mid 30s, improved steadily (and got injured regularly) my first couple years, kind of plateaued the next couple on 45-60 MPW, upped my mileage and frequency (to, during my better stretches, 80-100 MPW with doubles most days and slower but longer workouts) and improved tremendously (e.g. 8:xx off my half marathon time). No doubt a steady diet of decent mileage with the right workouts at the right times would keep me improving for some years, if I make that a priority and life doesn't get in the way, as it did for much of 2007.
You've got a ways to go in terms of seasoning, experience, lots more miles in your legs and in your weeks... trust me, you're nowhere near done learning how to train yet, let alone maxed from years of optimal training.
Sources for "7 years"? Some books (Fixx, I think Glover and/or Noakes, maybe others), some runners. Again, read people's real-life stories here and you'll see plenty improve longer than that. Bear in mind, some run eight years of HS and college and continue to improve for years after that.