On the reading bike wheel revolutions, your probably not going to be able to choose to do it once every 5 turns. More like once per turn with some kind of Hall effect sensor. Or more if you have an encoder. Recording data to an SD card is easy enough with the SD library. Easy enough to just write the current time, and number of revolutions so far, to the SD card as comma delineated, or tab delineated (programs like excel can recognize the comma and tab formatting as a spreadsheet and open them).
As far as comparing the 2 times of your previous rides with the current one on the arduino, while your riding... i guess its doable, but id say you want to get some of your data, and results into excel to work before going too far ahead of yourself.