Workouts can not be used to grade fitness. A workout can be ran to easy or two hard and will not predict anything accurately. TT and Races are the only true ways to predict fitness, and even these need to be ran at max pace + descent conditions.
Now with that being said, your workout was done in a positive split fashion, which leads me to bealive you ran it to hard. If you even want to start talking about judgeing fitness from workouts you need to have even splits and or slightly negative splits.
Your heavy drop in pace indicates a fatigue weakness and this would rear it's ugly head in a 10 miler race.
Start doing 10 Mile cut downs to goal pace for the last 2 or 3 miles and 3 x 3 miles with half time rest. If you can not complete this workouts you need to re-establish goal pace.
Examples:
10 Mile goal = 1:00:00 = 6 min pace
Long Interval
3 x 3 = 18 Min 3 miles - 9 min rest x 3
10 Mile Cut down
7:00,7:00,6:45,6:45,6:30,6:30,6:15,6:15,6:00,6:00
Now completion of these workouts does not mean you can run your goal pace for the full 10 miles, but these are the type of workouts i use to put goal pace in perspective for my athletes.
If you fatigue yourself with LESS mileage/pace then in the race, and you can not complete the end of the workout at goal pace it is an eye opener.