Choose a hilly or flat loop of medium distance and as many right turns as you can string together. Repeat the loop once a week or maybe twice a month and mark that in a "comment" field. Heart rate monitors are pretty cheap and a very effective tool for helping quantify performance as well. I used one for years before getting a Powertap. I also record a Relative Perceived Effort (RPE) in my Excel sheet. It usually matches the average heart rate pretty well.