Considering that your R is very large, the resistnace of the BNC cable would be negligable. Also, even though you are at 10MHz, you should not have to worry about RF effects, so you should not need to worry about 1/4 or 1/2 wavelength matching for an experiment with this simplicity.
What are you using to measure the voltage, a multimeter, an oscilloscope, etc.?
Most measuring equipment has an input impedance of something like 1 to 10 megaohms, so your large resistor value could be loading down your measuring equipment.