8 miles at 6:30 pace is far better for me than 10 at 7:30 pace. I know this from very recent experience.
I trained at the slower pace (7:30)all winter and mixed in a race every couple weeks. When spring came around I was not in very good shape. I was ticked off about my times so I just started hammering runs at under 7 minute pace. That lasted 3 days. I took a day off and did it again. Soon I was running hard 3 days in a row and taking a day off. I got down to around 6:40 pace before we started getting hot humid weather. When we got one cool day, I ran 10 miles right at 65 minutes. (33:40 out/31:20 back)
My race times kept coming down and I went from a 25:02 to 23:20 for 4 miles.