However many mile repeat you're thinking about doing, the last one should not be much slower than the first one without struggling like hell. If you can manage 5 repeat mile at 4:40; fine, go ahead and do them. And if you're expecting to break 27 minutes for 10, you probably should be doing them a bit faster anyways (I know you're not that kid though I would love to get his name revealed so we can all keep our eyes on the first American to go under 27). But if you can only manage 3 of them at 5:50 pace, so be it too. The point is; you cannot give yourself predetermined time and try to stick with it regardless whether it's too fast or too slow. If you can only manage 6:15 for the first time, that's fine too. But as long as you've built decent base, the times will come down naturally. How much depends on your background of training and the rate of adaptation. But, probably the most important thing, the point is; you learn from this experience so next time when you put the program together, you'll know what to do and how to do them. Most people try to do them too hard from the get-go and destroy their chances or get depressed and quit.
I've been training base since mid May. So now I am 13 weeks away from my focal race and I want to work in some interval stuff. I was thinking repeats milers next Tuesday with 1:1 rest. What kind of effort should I give these?