How far in advance do you start training for a marathon

I'm training for my first 10K next Saturday and my first half marathon in October. I plan to start training for it immediately after my 10K. I say, the sooner, the better!

It really depends on what kind of running base you have. Most training plans are about 10-12 weeks long. You want to make sure you don't increase your weekly mileage too quickly so you avoid injury. marathonrookie.com has a good half (and full) marathon training plan. I did my first half without following a training plan and felt like CRAP when I was done and got really sick post-race. The second time I followed a plan and finished with a PR and felt absolutely fantastic. FOLLOW A TRAINING PLAN

Most standard marathon plans are 18 weeks, but they assume you have a good running base. This means that you're running comfortably 3-5 days a week, and the first week your long run is usually about 10 miles. So if you're not to that point yet, you need to get running regularly (5 days is too many for me, 3-4 I feel healthier) and slowly build up to 10 mi at a time (increasing by 1 mile a week). If you are already there, 18 weeks gets you ready at a healthy progression without increasing your mileage too fast. If you do a half and want to keep training from there, I would not try to get away with any less than 12-14 weeks of marathon-specific training, especially if this is your first. I wish you a healthy and wonderful marathon training period!

ETA, I didn't read your post carefully enough and assumed you meant a full marathon. Sorry--I know much less about half-specific training.