After completing many marathons where I used an Excel template to track my training and running mileage, upon hearing an excellent Phedippidations podcast I began using a fantastic on-line training log called BuckeyeOutdoors.com. It's both extremely easy to use and offers many powerful advanced features.
For example, with simple web access anywhere in the world you can easily input your training activity or race to the site. Additionally BuckeyeOutdoors.com helps you easily follow your marathon training plan. Upon choosing one of several already existing on the site (or creating one of your own) superimposed over your training log is your training plan. Of great help is once you've completed your marathon, and decide to run another (it's inevitable that you will!) you can quickly re-set your successful plan to the new race date.
An optional, but highly motivating aspect of BuckeyeOutdoors is participating in 'Challenges'. Doing so allows you, in a friendly and encouraging way, to compete as a team member or individually, which provides motivation to run those miles.
Besides tracking running miles and time training BuckeyeOutdoors.com allows you to record your miles biked/swam and your strength workouts as well. BuckeyeOutdoor's creator, Ben Deutchle, is both an excellent web developer and as he's open to new ideas has consistently been improving the site even more.
Bottom line, for both marathoners and simle recreational athletes - check out the site!