Its a balance on this one, do you think you are capable of faster than 3:12, whats your goal ?
i'd suggest get your goal time is it sub 3? as a sub 3 plan this pace would fine.. and i'd suggest you would want one or two quicker..
some programs i've seen suggest an easy 10 miles at the start of the long run, then run the second 10 at a faster pace..
To, me 8:30 for 3:12 is too slow, I ran athens in 2013 but most of my long runs were 8 to 7:30 pace. i know runners that train 30 secs slower than race pace in long runs. I've put down my fastest marathon into the long run calculator on runners world and i believe its too slow, when i ran atherns in nov, i did'nt have the pace to sustain my goal and i feel it down to lack of long run pace..