Speedwork Suffering

I'm having trouble getting my track work done in preparation for the half marathon. The training plan today called for 7 miles total with easy warm-up and cool downs, 1600 meters x 3 @7:00 and 800 meter jogs between. I managed 1600 meters x 2 @ 6:53 and 6:55, 800 meters x 1 @3:27, and 400 meters x 1 @1:42, all with 800 meter jogs in between. I couldn't sustain what was to have been the 3rd 1600 meters. Not quite sure why I can't do this. The heat was tolerable today -- 69° and 71% humidity. I was well hydrated, drank Gatorade during each job, and had downed a Gu (chocolate) prior to running.

Maybe it's a function of age -- but I could do this 6 months ago. Perhaps it's because I swapped the easy week this week for next week's harder week because of vacation in the mountains next week -- did my legs (or my brain) just need the break? One consolation -- 3 weeks ago when I was supposed to do the same workout, I only did 1 1600 meter repeat and 1/4 of a lap on the 2nd 1600 repeat. I did improve.

SPLITS:
1.9993 mi x 17:49.75 (easy warm-up)
1600 mt x 6:53.09
800 mt x 4:23.76 (jog)
1600 mt x 6:54.88
800 mt x 4:45.79 (jog)
800 mt x 3:26.86
400 mt x 2:47.15 (jog)
400 mt x 1:42.27
1.9993 mi x 17:36.24 (easy cool down)

Total run: 7.48 miles at an average pace of 8:52 mpm.

Run well, y'all,
Bob