If you have a Garmin Forerunner and use the training center to track and view your runs, you may have noticed how limited the map is. In fact, I don't use the training center much and instead upload my maps directly to Trimble Outdoors, a GPS maps site for hiking, backpacking, geocaching, biking, running, fishing and boating.
I've written about using MapMyRun to find a run/walk previously, and while MapMyRun has a bigger database of runs to search through, Trimble Outdoors has a nicer UI with fewer disruptive ads and a nice map editor.
If you have the Garmin Communicator Plugin, you can upload your runs directly to your Trimble Outdoors account. After you're logged in, click on Map My Story in the navigation bar, then select Option 3: Import from a Garmin Device. Connect your Garmin Forerunner to your computer and hit Upload button. It only imports runs that you haven't already imported.
You can then edit your uploaded maps by clicking on My Account -> My Trips and select the trip you want to edit. The map tools are easy to use and allow you to add or remove data points (to modify the route), add photos, and add waypoints. It also has a ruler tool that lets you measure the distance between 2 points. Since the Forerunner names all the runs generically, I always update the name of the run/walk/hike to something that is more descriptive. I also add any points of interest that I think are important like restrooms or water fountains for runs, and views or sitting areas for hikes. For example, here's a Lake Merced run I've annotated on Trimble Outdoors.
Trimble Outdoors also shows you the elevation and speed/pace profiles in graph form next to the map. As well as giving you average speed and duration of your run. Most of this data is also displayed in the Garmin Training Center, but the UI is nicer on Trimble Outdoors. You can also export the map to your phone, or as a .gpx file, or to Google Earth. I use the Google Earth export to get a closer look at a new run or to look for parking if I'm going someplace unfamiliar.
Here's one method of adding waypoints using the Forerunner while you're running. If you have other suggestions, please share them in the comments.