Failed My First Tempo Run

Today was my first intense workout for Houston Marathon training:
10 miles with 4 miles at Lactate Threshold pace

My LT pace should be about 6:05 to 6:20. LT pace is usually estimated based on 10-mile race pace. I'd planned on running my 4¾ mile Dow course twice and then a ½ mile cooldown for 10 miles total. I was going to start out at about 7:30 pace and then run LT pace on the two Dow- miles going out, take a ¾ mile breather and go back to LT pace for the Dow- two miles on the return. But I didn't get onto the street until 0351 and with garbage day on Tuesday, I decided to cut it down to a Dow 4¾ mile warmup at near 7:00 pace, then 4 straight LT-pace miles on the Dow- out and back and finally a ½ mile cooldown. I figured I could finish that 9¼ mile run and make it back in time to get the garbage out, make my lunch, take a shower, and leave for work by 0535.

I had no problem running 7 mpm for the first 4¾ miles, but after the first half mile at LT pace I started to slow. I finished the first two LT miles at 13:11 and slowed all the way down to 13:41 on the return. I was pushing pretty hard, but I just couldn't get back to anything near a 6:20 pace. It was pretty warm this morning, 80.7°F average from 4AM to 5AM with the dewpoint in the mid 70s. There was a little rain on the last mile, but not enough to really cool me down. I know I can't run as well with temperatures that warm. My effort should have been good enough for LT pace, but I don't know whether you actually reach LT when you're forced to run slower on warm days.

My next LT run isn't until the 9th of October. I'm hoping we'll have some real cool air down here by then.