Stuart, like yourself I'm going for a second marathon and also aiming for 3:15. I've done quite a bit of reading and background research and majority of opinion always suggests not to do your long runs at race pace but at least 30-45 secs slower. I tend to follow Hal Hidgons schedule with a few adjustments to suit my own routine. I try to do a Saturday sorta long run at race pace (no more than 10 miles) and the do my Sunday long runs that bit easier, trying to get my HR down lower. I then throw in a speed session and a tempo run during the week.
I'm currently only doing 14 miles for my long runs at the minute, but intend to (and I always find this hard) to slow down a bit once I start building upwards from 16, with the aim of doing one long run of at least 3:15, but only covering about 22 miles, just to get myself used to being on my feet for that long.
Bottom line though is to make sure to get to that start line with no injuries but enough confidence that you've done enough training.