Guitars like to see a higher impedance. A pair of 1 Megohm or 2M resistors would be better. With a 50k load (a pair of 100k resistors) the signal is going to get knocked-down a bit, and your tone will probably be affected.
With a high impedance load and the guitar volume cranked-up, you should get around 1V, which should be plenty of signal to work with.
With audio inputs, I normally use a peak detector
(You'll need to change the 100k input resistor to 1M or more.) A peak detector will work with lower signal levels, and since it's not biased at 2.5V (like the 2-resistor method) you can use the 1.1V ADC reference with weaker signals.