Ok, well I can tell you at Rome I ran the first 20 miles at an average of 9:08 m/m, and the last, ahem, 6.7 miles! at 8:46 m/m. Similar on my pb, though that was 4 years ago now, first 20, 8:30 m/m last 6 8:14, both races felt really good energy wise and cardio wise. Previous to Rome I did Luton last year, I went a little crazy at 13 miles ran the first half at 9:05, second half at 8:43, which I paid for with calf cramps by 20 miles.
Definitely do not go out like a rocket. Either keep to those 10 m/m or shave some off but I wouldn't recommend shaving off more tha 10-15 seconds a mile, unless you've really trained for that pace or the last race pace felt easy.
I can honestly say I was held back in Rome by my hamstring, my cardio felt really easy the whole way round except maybe the last couple of miles. But my hamstring may have been my saving grace, had my legs felt fine I may gone off too hard and struggled in the latter stages.