8 seconds will take about as much power as 1 second interval. At least 1/2. It takes time to initialize the GPS on powerup. If you really want to save power record a data point every 2 minutes. You might get the average down to 1-2ma in this case. http://www.adafruit.com/products/746
can run without a microcontroller taking power at all. It only draws 20ma. But I do not recommend this. Because then you will have no control over the period or to stop sampling when there is no movement. It is also difficult to access the stored data with Arduino code. If you store the data in EEPROM, you could store 500 data points. That's 1000 minutes or 16 hours of hiking. if you want 3 days you will have to sample less often or use SD card. This is what I'm working on.