I agree that lots of miles seems to make a dramatic difference. I came within a whisker of 17mph (moving) average over a 300km brevet at Easter - this is with no drafting, of course.
Unfortunately it's a bit of a risk jumping in on the big miles. I thought I had built up fairly steadily, from ~200 miles in January to ~700 in May but it seems this was too aggressive for my knees, which took the opportunity of some cleat misalignment (or possibly the boom not being in straight?) to explode horribly on my last big ride
I'd say for someone who rides regularly already, you should think in terms of 1,000 miles to adapt, not a hundred or so - and for me, it was nearer 2,000 before I felt quite happy.