Not a Good Sign for Sunday's 10-Miler

Today was my second intense workout for Houston Marathon training:
10 miles with 4 miles at Lactate Threshold pace (same workout two weeks ago)


I started a little late again, so I cut the run down to two times on the 4¾ mile Dow course with four miles at LT pace on the return Dow- two miles and the second two Dow- miles going out.
I hit the first LT mile at about 6:20, but I was hot and tired and couldn't hold onto pace. The first two LT miles were at 12:57 and the 3rd/4th were at 13:21.
I was hoping to run Sunday's 10-miler in the low 60s, but that's not likely when I can't run four miles much faster than 6:35 pace. It wasn't too bad this morning, 78°F average temperature from 4AM to 5AM with the dewpoint in the mid 70s. Maybe it'll be much cooler Sunday. That's my only hope.