Break it down.
That is the idea behind agile - break down large software projects into digestible chunks and execute in stages.
1. What are the big components of your big task? What are the smaller ones?
2. Is there a natural order of execution for those pieces? What are their priorities to you? Which ones you could tackle on your own and which ones you would need help with? Which ones could you measure impact on quickly and determine ROI on?
3. Are there time-consuming parts that are not critical to your business that you could outsource or delegate? If delegation will require effort and time to define task, scrap the idea of delegation unless you would save a lot of your time down the road because the task is ongoing.
Look over the pieces, pick the one you have sufficient skills on and can also evaluate results on fairly quickly/easily and start scratching. Any small measurable successes will inspire you to tackle more of the tasks and break out. Nothing is as motivating as seeing results of your work.
p.s. If you need help identifying the right marketing activities for your business, post another question with some basic info on the business (type, stage) and I am sure this community will help you come up with some priorities and share online resources as well.
Before even dissecting the task into smaller pieces, you should understand why you are doing it. The core reason being it's very easy to create "work" even when it doesn't affect the bottom line.
So first see if it:
You'll be far more persistent in getting it done once you've ironed this out.
Then you start with breaking it down into smaller tasks. I would recommend reading this summary of GTD by David Allen in case you haven't already.