When I was trying to get fit for skiing last year I was going out on the rigid hardtail and every time there was an uphill slope on the road I would shift to a gear that would allow me to stand up and spin the pedals - which would eventually lead to a lactic burn in the thighs.
After several weeks of this I was finding that I wasn't making much improvement and was still getting as much lactic burn.
So in the final week before skiing I ran 7 miles each day for 3 days, a day off, then another 3 days of 7 miles, and that got rid of the lactic burn onset.
But the point of the story is that normally 2 days of consecutive 7 miles runs are enough to make my knees ache, but not in this case and I then lasted a week of off-piste skiing before my knees ached.
So I think the spinning on the bike - where I was out of the saddle and spinning at maybe 70 cadence and not grinding so having to support my weight on both legs - stabilized my knees by toning up all the muscles around them - and my running was a lot more 'bouncy' than normal.
So if I were to do a marathon (I have only done 2 1/2 marathons) I would be considering mixing up my training in a bit more intelligent way than just running.
That would include interval runs of 100 paces of jogging and 50 paces of sprinting and then gasp and repeat, hill climbing out of the saddle on my road bike , and some 7 mile runs with the occasional longer one, and gradually build up the distance I could run.
And I am not sure consecutive days of running are necessarily a good idea so I would stick to a day of rest between.
I over-trained for my first 1/2 marathon and did too much running and it was a long while before I lost the niggley pain in my right knee.