Why am I running slower?

Not the end of the world, but an interesting problem that I would like some opinions on-

I had been runnnig a 1:48 half marathon, and my pace on most of my sprint tris run portion was in the 7:20 to just under 8:00 pace - depending on heat/humid and hills. Not great, but not too bad.
Swim and bike parts always much better, so I generally do ok for a 46 yr old guy.

This past fall I started some longer distance runs, getting ready for a marathon. My long runs went from 10 miles to eventually a series of 20 mile runs. My long runs were at least 13 to 17 for much of the fall and early spring.

So here I am in 2009, my endurance is better, but I just don't seem to have the speed. A recent sprint tri had my run pace at almost 8:35 and a recent half marathon was about 1:53. My 10k time is also a few minutes slower than it had been on the same course.

To add emphasis, when I finished the sprint tri where I could go no faster than 8:30, I felt so good at the end of the race, I jogged back thru the run portion to warm down, but almost at the same race pace. I still had plenty of fuel in the tank.

So now I wonder if was possible to build endurance at the cost of speed? Or maybe I am just getting older and don't have that kick? : )

What do you think?