If you are going to run a marathon, it is important to carb load the day before. 
Is it good to sleep a lot?
Since you will be running long distance for more than 26 miles, than you really should get good sleep
How popular are marathons?
very, there are more than 800 marathons held every year. 
Do they do them if it's raining?
I think it depends on the event and how bad the weather is. 
Can you walk during it?