caveat - I know these are pedestrian numbers (go ahead, ridicule away), its the unexplained rate of change that is confounding. I'm wondering if I am going to burn out or hurt myself or if something is just starting to click. I really dont know.

I started running regularly in 2008. I started riding regularly in 2009. Aside from injury time outs, my training has been pretty consistent 30-50 miles per week, more towards 30 over the winter, more towards 50 over the summer. My weight has been really steady over this time period and while my run pace has been pretty consistent over the last 4 years, I do have to say I have been getting stronger on the bike.

My half marathon PR history is:
10/2009: 1:51:00
11/2011: 1:50:30

I ran regularly over the winter laying the base for a HIM planned in June (I've already got a couple under my belt)

What I cant explain is this -- a 6 minute improvement over my PR @ a well below threshold level of intensity, but more than that a 14 minute improvement over just a month earlier:

Race (half marathon) at the end of February:
Conditions: RACE
Temperature: 35 degrees
Elevation Gain: +158 ft
Chronic Fatigue Level: Tapered and Rested (Friel TSB: +18)
Acute Fatigue Level: 7 hours of sleep
Body Weight: 219
Avg HR: ~160 (172, 1 mi LTHR)
Time: 1:58:00

Random 14 mile Training Run 3/31/12
Conditions: Training Run
Temperature: 35 degrees
Elevation Gain: +408 ft
Chronic Fatigue Level: Fatigued - (PR'd a 10k 3 days prior, 20 min TT @ local bike shop 2 days prior, rest 1 day prior (Friel TSB: -30)
Acute Fatigue Level: 5 Hours of sleep
Body Weight: 218
Avg HR: 155 (172, 1 mi LTHR)
Time: 1:44:22 (for 13.1 miles of it)

So again, I know these are pedestrian numbers, but its unexplained rate of change that is confounding. Am I going to burn out? or hurt myself? or is something just starting to click? Has anyone else experienced these kinds of breakouts before?

Thanks in advance for the perspective

Maybe you had a crappy day at your race and a great day on your training run?

What time of day were each of these runs?
What about your pre-race and in-race nutrition / hydration? (Maybe at the race, since there was an aid station every few miles, you stopped to grab something and it slowed you down).
Did you stop to take a piss? Fix your shoe? Hug a spectator?
Were you sick at all leading up to the February race? (I always catch a cold in January).

I have no input, just wanted to congratulate you on the big PR. That is a great improvement. Stay injury free and drop some weight and you will be sub 1:40 soon. Real soon. Like 10 pounds is all it would take soon.

36. agreed they are soft PRs, but they were PR's based on all out threshold efforts. so regardless of the base, they were all out (as referenced by the HR as a measure of effort). And you're right. If I didnt eat so damn much, I'd be faster ... but that isnt the variable in this case. No speedowork -- just tempo based intensity. I've felt like I wanted to focus on getting a volume base of fitness through mileage, rather than beating myself up with speedwork. Having said that, I do tons of speedwork on the bike.

You could probably store that much glycogen in your muscles and liver, so there's not as much of a need to try and replace calories burned during a half marathon. But if it works to keep you going then stick to it. As your time decreases, you may not be able to stomach as much so keep that in mind.

Thanks. Do you know how long I've been trying to lose that 10 lbs?
Now if I could drop the 10lbs and run a sub 1:40 as the 3rd leg of a half iron, I might just opt for lipo ...

Have you tried a food log along with your mileage log? If you start tracking your food in a log, you quickly realize how many empty calories those little snacks have. I had a oreo habit, but after logging those into trainingpeaks once or twice, I felt like such a moron, I kicked the habit. There are probably a few little things in your diet you can tweak and be there. The quickest path to improvement is leaning out.