on an iphone, if you set a SIM passcode and restart the phone without unlocking the SIM, then you've turned off the cellular radio. You can turn bluetooth and wifi off manually. With this setup, an iPhone battery will last a good long while for doing occasional lookups of your position, but it doesn't have enough battery to be turned on consistently as you ride.

Seen this link?
http://kentsbike.blogspot.com/2007/01/tobys-coroplast-panniers-fenders.html

Train for power in the legs and recovery? I once rowed, and an effective training was running stadium steps. Find a football or soccer stadium. Run up the aisle steps, cross to then next seat section, gentle jog down, across to the next section, run up, repeat. An alternative: running up stairs (internal, fire escape) in an office building. Brutal training, but effective. As with any hard training, don't do too much too fast. Work up to it - eventually a whole stadium.

Easier said than done - i should follow my own advice!

Cf