Take a day off.
If you've not had a rest in weeks and your legs are dead, doing as little as possible today will be the most effective training session you can do. Any running will be straight in the trashcan and tomorrow's training won't be much better. Give your body 24 hours to take stock of the training you've done over the last few weeks and build on it. You'll be ready to go out and train twice as effectively tomorrow as you would have been otherwise too.
If you still feel the same tomorrow take another day off, no seriously! It'll make you feel sick with guilt and worry but realise that the body is effectively coming to terms with what you've been throwing at it and preparing itself to go up to the next level. Often I've taken several days off in the past after hitting a bit of a wall and more often than not I'll post my best and fastest week's training of the season straight after.
It's not about being a soft cock or a hard nut, it's about doing what's best as an athlete. Don't become a slave to your training diary, you won't be the best you can by filling every page. When your body sends you messages listen.