Alan Couzens, MS (Sports Science)
You'll find below a simple calculator to help you calculate your own fatigue curve (along with an estimate of your FTP)
To use, enter 2 or more best power numbers (from true max effort tests) for any of the durations listed - 5min, 20min, 60min, 2.5hr, 5hr, or an Ironman in which you ran well. Enter these in the white cells & clear the sample data from any cells that you don't have current data for. Ideally, these will be normalized power numbers (especially for the longer durations).
The curve produced will give you an estimate of FTP (the first number in the regression equation at the top of the chart -- y=....) and it will give you an index respresentative of your current 'fatigue curve'. For a 'key' on what this number means, scroll to the table in the middle of the article linked above. As a general rule of thumb, your % drop off each time the duration doubles will be ~2/3 of this index e.g. if that superscript # is -0.10, your fatigue rate is ~6.7% (0.10 x 2/3).
By tracking this 2 factor model over the course of a season, you'll not only gain a good sense of how your general aerobic fitness is improving (via the estimated FTP) but also how your strengths and weaknesses as an athlete are developing (via the fatigue index).
Happy data mining! :-)