ITBS that get tight after high mileage is normally caused by weak/tight misfiring glutes. Your ITBs are getting tight to compensate for them. When tight they can 'pull' on the knee causing the knee pain.
Running on outside of feet could also be a clue that your gluts and/or pirifomis muscles are tight. Get massaged & stretched by sports massage therapist.
Substiuting running mileage with glute strengthening routine (lunges & 1- leg squats) should help. (I did same when training for my first marathon) You can feel a big difference in two weeks so worth starting now.
Intermittent 'low-level' back ache is normally caused by tight muscles in your hips. Glut Medius, TFL and Rectus Femoris muscles are the common culprits (possibly hamstrings and/or glut Max) - try stretching both sides 3 - 4 times a day 1 min each until symptoms subside. I suspect a good deep tissue massage would help as well.
As DA said, could be general inflexability. In the gym try incorporating more dynamic cable/kettle bell/medicine ball exercises into your routine to help with general flexability.
HB - Progressive increases in distance or speed when running (not both at same time) will gradually re-strengthen the muscles. When recovering from a muscle injury "No Pain = Gain".
JC - Pain below calf muscle belly and above heel is probably achilles tendonosis - if injury more than a week old. Eccentric calf 'lowers' exercises are best for that. Discomfort while doing tendon (not muscle) rehab exercises is now thought to be a good sign (but not pain day after exercises).
I'd advise icing achilles on a stretch after exercise. When you’re sitting down, ball of your foot on coffee table and drop heel for a GENTLE calf stretch for 5 mins. Use ice* to rub up & down both sides of achilles as ice melts. Keep it moving on the skin or risk an ice burn (*hold cube in tissue or use a polystyrene cup full of frozen water and tear down as ice melts*
Hang your heel off table - don't be tempted to push heel down - doesn't need it and may only irritate the tendon. Keep relaxed
If pain is relieved by stretching calves/hamstrings it probably is cramp. Rehydration after running is key. Lack of mineral salts - potassium & sodium is also linked. Generally tight muscles can also cause night cramps. Make sure you stretch calves and hamstrings properly after runs and before bed.
In first instance I'd agree with wonderwoman. Sciatic nerve impingment will give those symptoms. If its intermitttent then more likely to be muscular cause. Tight left piriformis could be impinging on nerve. Could be getting tight/or stretched! after sitting because of antagonist muscles on right side - Tight glut medius or TFL muscles on right hip??
Have had clients with simliar problem. I would try stretching out right hip first...