When we're training for an event, it's easy to get caught up in wanting to do lots of speedwork in order to improve. Speedwork is important, don't get me wrong--you have to go fast to be fast. But the majority of your running should be at a much more sedate pace.
Keeping the majority of your miles easy will serve a few purposes. For one, it helps stave off injuries. For another, it trains your body to tap into fat for fuel. And you know what? Most of us have enough fat on our bodies to sustain for an entire marathon! But all that fat does you no good if you haven't trained your body to access it.
How much time should you spend doing speedwork? Assuming you have a good base under you, speedwork should account for about 10 percent of your weekly total. So if you're running 30 miles per week, a safe amount of speedwork would be 3 miles per week. The more experienced you become, and the more miles you add to your base, the more time you can spend at faster speeds, within reason.
The rest of your training should be at a more relaxed, fairly conversational pace. On long runs, aim for a pace a minute to a minute and a half slower per mile than your marathon pace. Don't know this pace? One easy formula for determining your pace is to train at 80 percent effort or less--80 percent of the pace you could run the same distance in a race (assuming that's 100 percent).
So the next time you're tempted to pick up the pace on your long run with buddies, hold back a bit. Your running will thank you in the long run.