Running On Empty

Since I completed the C25K I've maintained the habit of running three days per week. While my speed remains the same slow jog it was back at the end of week nine, I still manage to complete the full half hour. So I was very surprised when I barely made the fifteen minute mark today.
Even though I had set out at my accustomed leisurely pace I started to feel tired at around the ten minute mark. Believing the fatigue to be mental rather than physical, I plowed on thinking I'd work through it, until just past the fifteen minute mark where I well and truly hit the wall. I literally couldn't run another step. I stuttered to a walk and scanned myself searching for the problem.
I was mildly out of breath but certainly not gasping or panting. I'd worked up a light hairline sweat but didn't feel overheated. The usual suspects (feet and left knee) were all in good order-no pain or stiffness. Even my left hip didn't hurt any more than usual (it often aches while I run but the exercise seems to loosen it up and it won't bother me again all day).
Finding nothing overtly wrong I picked up the pace, but only to a very slow jog, and a couple of minutes later was back to walking. I'd be surprised if I actually ran more than six or seven of the following fifteen minutes. The entire experience was quite disheartening.
Any number of factors could have contributed to the exhaustion. I didn't sleep particularly well last night. I was inappropriately dressed for the weather (it was overcast and threatening rain when I left so I wore long warm pants but once under way I discovered it was warmer than it looked). Conditions were warm and humid, not typical of this time of year. All of these factors may have contributed to my lethargy.
But I strongly suspect that the kick start diet played a large part. I've been following this very low calorie diet for over a week now and though I have ample fat stores I think the exercise was just a bit too much for me at my current intake level.
Maybe this was just my first 'bad' run. I've heard they happen to everyone occasionally. But the timing seems a bit too coincidental.
It'll be interesting to see how my next run goes. I'll report back later in the week.