Are you doing loops around the park? It seems to me the game periodically queries your coordinates, and determines your distance by calculating the difference.
The sample rate seems to me to be more than 1 minute, maybe 2. I'm really not sure why it is so infrequent, but it is.
In an extreme example, if it checked your location every two minutes, and you happened to complete a lap around the park every 2 minutes, the game could assume you had not moved at all.
If you intend to go 6 miles, your best bet is to go out 3 miles straight, and make sure that distance is captured before embarking on the return leg.
ETA: I posted a few pages back, I rode my bike 11km out and back; and less than 2km registered each way. I think the game also throws out any lengths where it determines you exceeded a specified speed (15mph?). Because my 11km route was mostly straight. Another factor which I am not sure about, is whether it references your device's pedometer.