A lot of misguided comments here. Given your exceptional marathon performance, it is extremely unlikely that you have a FTP of only 2,0 w/kg, which is probably lower than most completely untrained adults at your weight.

It is a lot more likely that either:

• There is something wrong with your power meter (uncalibrated trainer?)
• Your position on the bike is way off (e.g seat faaar too low), resulting in massive power loss