Speedwork for the Young and Old

Most people I know that run say that long runs improve your endurance while speedwork helps you run faster.  I've been running for over twenty years now and for a number of those years I kept a log of my workouts. I recently decided to pull out my 1997-1999 logs and compare the speedwork back then with what I did from 2008 -2010, and in particular, for ten consecutive weeks last year.  I've felt that last year's speedwork block just didn't produce the expected results. Did I do something different last year that hindered my results? Perhaps it's just because I'm ten years older now or maybe there's something more to the story.


Stats:

My 1/2 mile repeats back in 1998 and 1999: Range (2:44 to 3:00 minutes)-12-13 workouts per year
My 1/2 repeats from 2009                          :  Range (3:14 to 3:35 minutes)-10 workouts

Note: I did mile repeats 2 to 3 times in 1999, but not in 1998 and 2009.

Mileage in  1998 and 1999: 40 to 50 miles per week
Mileage in 2007                :  35 to 45 miles per week
Mileage in 2009                :  30 to 35 miles per week

I was training for fall marathons in 1998 and 1999
I trained for the New Haven 20K in 2009

Manchester Road Race: 1998 (4.75 miles) 30:35
"        "        "        "       : 1999 (4.75 miles) 29:31
"        "        "        "       : 2007 ( "   "    "    ) 34:23
"         "       "         "      : 2009 ( "    "    "   ) 35:11

I'm going to ponder these results for a while and give you my impression later this weekend. What do you think?