I am not a doctor. 

First thing I do when my knees hurt is buy new shoes.

Ice on the location of the pain is good to reduce swelling, numb the area and encourage healing.

I generally take advil for pain during marathons, but you need to make sure you don't drink too much fluid.

You should try to see a sports orthopedist or just plain a plain orthopedist.  Someone who specializes with athletes, especially football players, is the best as far as knee problems..  A doctor who is also a runner is great.

You don't mention swelling, redness, popping or cracking sounds.  Sounds are bad.  You oviously didn't dislocate the kneecap which is very bad.

You don't describe the running conditions when you injured it.  Trails, down hill, up hill, and speed training are all hard on knees.

You don't mention your expected marathon time and how often you run marathons.

You don't describe the intensity of the pain, but if you are still walking on it the it might not be too bad.

Make sure your stride length isn't too long.  Many runners have the mistaken impression that they have to increase stride length in order to run faster.  To run faster you have to move your feet quicker, stride length changes naturally as you get stronger and faster.

I think if it hurt bad enough you would see a doctor.  It may not be too bad.

I'd run the marathon, but then I'm not very bright.

Good Luck

You sound similar to me.   I have run a lot more marathons though.  My goal is to finish without injury since I always have one more soon.  My times are 4 hours give or take, 44 years, marathons for nearly 12 years.

I've been taking a Glucasomine/Chondrotin combo pill for about two years once a week to try to prevent joint problems.  For while it always seemed like I had a twinge of knee pain after taking one.

I wear ASICs Gel Cumulus with "mysole" inserts.  The inserts helped with a vicious bought of PF in both feet that I had for a couple of years following a car accident and the inserts give arch support for my pancake flat size 15 feet.  If I don't have knee trouble I wear my shoes until the soles fall off.  I just through 3 pair away last week that I had worn for over a year and a half with lots of miles and marathons.  I usually cycle 3-4 pairs at a time.  I never usually wear the same pair on consecutive runs.  But I may have a favorite pair for marathons.

The constant trail running could be contributing to the problem.  How do you feel when road running?

Maybe you should go back the cheapo shoes?

Age might be contributing, but what I have read says that runners don't experience joint problems unless they really abused their joints when they were young.  Elite athletes seem to need replacement joints but us slow guys are good until...?

I have a hamstring strain that's been bothering for over a year.  Darn thing won't heal and I won't stop running.

My advice, forget the PR and take it easy during the marathon.

I hope it works out.

A guess here.  With all your other injuries and sore spots you may be unconsciously favoring them and throwing off your gait causing other tender spots.  My first thought was IT band issues.  They show up on the outside of your knee.  You may want to try rolling with the foam roller on the outside of your leg, both upper and lower leg.  You mention shins having a problem.  If everything is tight along the outside of your leg you need to loosen and stretch over there.  If the shoes are shot that may cause the foot to wobble a bit more and overstretch those muscles.

For the person with the patella femoral syndrome (knee cap to the side), you need to strengthen your vastus medialis muscle.  Google it.  It comes into play in the last 15% of extension meaning when your leg is almost straight.  Sit on the floor with the leg over something like a rolled up thick towel so your leg is bent slightly at the knee.  Pick your foot up off the floor by extending at the knee so you are extending your leg all the way out straight.  Gradually add a little resistance so the muscle get stronger.  Hope both of you can get back to running soon.