I wouldn't rule out poor aerobic conditioning.  My "return to running" after my Achilles rupture in 2010 really only started in April 2011 and I've only been really seriously training since August 2011.  Given that, there's probably a lot of work still to do on my part before I stabilize at a lower rate.  I do know that I'm still seeing significant endurance/speed improvements right now so until Thunder Road is done I'm just focusing on getting the mileage in my training plan calls for, but after that I'll be investigating long term physiological goals and changes.  (I've also been reading about stress, caffeine, and partial dehydration increasing HR and these are things I'll be looking to address soon, too.)

Also, for those reviewing past log data, be aware that HR data prior to 10/8 was on my old Garmin 301, and I noticed the occasional problem with the chest strap sensor futzing out on me with that unit (I'm sure that's where the 240 bpm data came from).  So while I think it was MOSTLY accurate, I don't have too much confidence in specifics with that unit.  Since 10/8 I've been running with my new Garmin 610 with a different heart rate strap and I'm much more confident the values I'm seeing are accurate through the full duration of the workout.  Still high, but accurate.

I just plotted max HR for the past year's workouts vs. date to see what the scatterplot looked like.  I'm still not sure I can trust any of the data prior to 10/8, but October/November seems to be providing a lot closer grouping and a couple more months of data should let me know if I'm holding steady or decreasing there.

Of course, average heart rate for the same period doesn't show the same skew with the old monitor, so it's possible only the max hr was affected with the 301 unit and I AM holding steady with my average HR.

Yay... got my PR at the Vulcan Run 10k... 41:47!

Now I gotta drive 400 miles home! ... Totally worth it though!