5'10 pace is a lot faster than your goal race pace for sub 70 (5'20 pace). When I was training to run sub 70, I would do some HMP work at the end of long runs to practice running that pace on tired legs. I'm not sure why you would want to run 10sec faster, especially on the first ones.
I'd say the first 3M should be no faster than HMP (so 5'20s). This should feel very easy. Then do your 10M at normal training run pace and then finish with 3M at HMP. It should feel harder to run the same pace, assuming you don't take fluids or anything. If you're feeling good, try to run the last mile closer to 10km pace (which should be about 5'00). This will still average out to slower than your 5'10 pace 3M, but will simulate the pace you'll actually be running.
Good luck!