I've only done one marathon, but while training for it I followed the training plan the marathon organisers put out.
As a general rule, I disagree strongly with some of the advice in the current top comment. Your longest run should cap out at about 35 km, definitely not the full distance. And you should taper. Your longest training run should be a couple of weeks out, and your final week should be more focused on shorter efforts to keep your sharp, rather than on volume. You want to be well-rested for the big day.
20 km weekly is very low volume for a full marathon. Initially and during the taper that's fine, but at peak load you should probably be doing about a full marathon across the week (but not in one run!). Though perhaps this is something that depends on level. If there's anything in my comment that doesn't apply at all levels, it's this.
Run/walking is also perfectly fine if you need to, especially to prevent injury. It's not a tactic that's going to get you a sub-3hr time, but that doesn't mean it's bad. Listen to your body. Joint pain and sharp pain like shin splints is bad and you should stop or slow down. A more aching pain in your muscles might be something you can continue through.