LS, the accepted wisdom is that long slow runs (not ALL your runs!) should be run at 2 minutes per mile slower than your projected marathon pace.
Like you, I find it quite difficult now to imagine running mile after mile at 11-minute-miling. Today's run was a recovery run (which I think it's OK to do at 90s slower than intended race pace, though I could be mistaken) and the race I'm actually training for now is a half-marathon which I'd like to do at around 8-minute miling, so ... excuses, excuses!
The main benefit of the slower pace is that you run within your aerobic zone and it provides better cardiovascular training. It's also easier to sustain for longer, and there's a lot of psychological benefit to be had from learning to be out on your feet for three hours at a time. And it IS possible to do long training runs at 11-minute-miling but race a marathon at 9-minute-miling, provided you include some shorter, sharper runs or structured speedwork in your schedules.