I have done a few marathons and found that the 20 mile mark was a killer for me in both instances but I recovered after a few miles. I found 1/2 marathons pretty easy while I was training and was surprised by how much harder the full marathon was. I think if I were to do it again I would find a training group that intended to try to run the pace I was shooting for so that I could become comfortable with a set pace. I ended up right around 3:40 for both the marathons I ran, the first one I was on pace for 3:20 at the half and the second one I was on pace for 3:40 at the half, I will leave you to guess which one hurt the most.

In both runs and in my training I had more problems with my feet than any of my joints, you really need to be sure that you have a known comfortable pair of socks and shoes for your run. I made the mistake of replacing my shoes just a few weeks before the first marathon I did and I ended up with some pretty bad blisters. I did end up with some knee pain after my first marathon as it was really hilly (Big Sur) and the downhill portions were tough on the knees but it only bothered me after the run.