Originally Posted by
Juha
The "snapshot" can take several minutes. If you move for 15-20 minutes with the GPS turned off (this seems to be what you want), the receiver has to start searching for satellites again. That time can be reduced with wifi / GSM cell tower triangulation, not sure how much iPhone makes use of those.
I don't know how smart the initialization sequence is, but if the phone was locked 20 minutes ago and you are moving a biking speeds, my guess is the subsequent triangulations would be much faster.
Another thought for the OP is to buy an external battery to plug into the iPhone and just record your GPS track regularly. If you can recharge at night, then you should be golden. (If not...)