Dead Legs

I need some advice...thinking we are doing something wrong. Sunday was a 16 miles and it was a long and hard 16 but not the worst run ever. We took yesterday off and ran this evening and it was pretty close to terrible. We have this pattern where we have a good week, ending with a long run and then suffer terribly for the next week. In an ideal world I would run hard for a week and recover for the next week, that's not how we do it. We run hard for 3 weeks and recovery for a week. After the 16 miles I stretched, worked on drinking copious amounts of water over the next 24 hours but was still feeling awful tonight. Nothing in particular just very dead feeling legs. We were able to get in the 6.5 but I'm wondering how others feel after these longer runs. Over the next 6 weeks or so we will be tackling several long runs and I would like to keep my miles up in between and not suffer so much. Don't get me wrong it feels better than it has during previous marathon training but I think it could feel better. Maybe I'm delusional and it's awful for everyone it will make me feel better and if there are some secret tips that will help that's good as well.

Days till NYC: 67
Miles trained today: 6.5