The answer to your question depends on your goals. If you just want to finish a marathon, you can do it without doing any speed work. If you're looking for the best performance you can muster, then speed work will help a great deal.
There are varying degrees of speed work. Varying pace and distance of the repeat stresses your body in slightly different ways and improves different areas. Short (less than 2 1/2 minutes) efforts run at about your mile race pace will help improve turn over and efficiency. Efforts lasting 2 1/2 to to 5 minutes run a bit slower than your 5K race pace will improve your Max VO2 (maximum rate your muscles can absorb oxygen from your blood). Runs lasting from 5 to 20 minutes run a bit slower than your 10K race pace will help increase your lactic threshold and help you get used to running quick for longer periods of time.
For the marathon, I think the order of importance of these speed workouts is from long to short (i.e. concentrate more on the 5-20 minute stuff). You can do them either on a track or out on the roads as a fartlek.