My impression is that Forerunners have fallen out of favor. The GPS doesn't work all that well compared to a phone (I'm guessing because it lacks the AGPS data a phone has access to). Plus if you're running a marathon you probably want some music to listen to, and it sure is nice to have a phone with you even though it's perhaps a bit heavy.
Moves iPhone 5S - http://imgur.com/kjYf8Tj Garmin 910XT - http://imgur.com/jnXuaQC
That's what RunKeeper recommends, because the AGPS using WLAN-SSIDs lowers the accuracy of the tracking.
One might look for a higher power setting if you are doing an hour / half hour run and not concerned about keeping battery life up.
Before I put my running stuff on, I turn on the watch and let it figure stuff out while I'm taping my nips, and if it's winter, gloves, tights, mentally bracing myself for the cold, etc.
[1]: http://www.economist.com/news/international/21582288-satelli...
GPS satellites have a signal strength on par with a night light's, so it's easy to overpower in a small area using a low power transmission.
Once the almanac is downloaded, the phone's GPS track should look much closer to the Garmin's.
If this was a recurring problem then I don't know what to say, other than it doesn't match my experience with RunKeeper on an iPhone 4S.
It's a shame, but the most accurate Forerunners are the older ones–same for Foretrex, etc.–which are often the ones with the shortest battery life.