I think you've gone through the worst of it. I loved my 15 mile+ runs when I was training for the FLM and actually missed them now when I went back to 10k racing. Before you know it you'll be tapering off so try to enjoy those long ones! Besides, you may never run at that distance again and it is such an achievement to know that you can run 15/16/17 miles off the cuff. You won't even realise how fit and motivated you are until you try to retrain to that standard - like me now getting ready for a half marathon and struggling!
Good luck x