26 miles at 10 min miling is 4:20 marathon pace, which is slap bang in the middle of your target range! In effect, what you're trying to do is a 10 mile run at marathon pace; it's no wonder you're knackered! Have a look through your training plan and see when it asks you to do 10 miles at marathon pace; not yet (if ever), I would suspect!
The 'S' in LSR stands for slow! Matty's right with around 11 min miling. It should feel easy; that's where your race day endurance will come from. Race day speed comes from rthe other sessions (honest!)
1) there's plenty of time to get said toothache looked at before Sunday; google Dental Access Centres in your area and don't put it off!
2) Ibuprofen is very good for dental pain, but it probably won't stop the problem from going from reversible pulpitis to irreversible pulpitis to apical periodontitis to a full-blown abscess without some proper treatment.
3) Raising your blood pressure will also raise the blood pressure in the tooth, which will make it hurt more
4) To be honest, taking either paracetamol and/or ibuprofen before your race will probably do you no harm at all, as long as you're sensible with fluids and nutrition; just read the labels first
5) The safest thing to ensure your short term survival is to stay in bed and not race at all, but please don't fear death. It's coming to us all one way or another. Getting to the race is most likely far more dangerous than the race itself... though I have no statistics to back this up.
(I am a dentist; flyers is a vet and knows her stuff, though I think patient management for vets mostly consists of 'hold Flossie down firmly' )