18 weeks out from the Dublin City Marathon, and I find myself talking repeatedly about the 10% rule- increasing your training volume by more than 10% per week increases your risk of injury massively (see the image - >10% increase more than doubles your injury risk). If you are planning on doing the marathon, your training is most likely going to start within the next few weeks. Choose a sensible programme matched to where you're at now- not where you'd like to be!! Just because you want to run a sub-4 hour doesn't mean you can this time around -if you pick a programme thats too hard, you'll probably just get injured or sick, so be realistic.
Bear in mind you need to already be running regularly to be able to start one of these programmes- most of these start on the basis that you are already running a minimum of 30km per wk.