The motor is designed to be used under usual and reasonable conditions. 20 miles per hour is usual and reasonable. There are a ton of variables that you have left out of your inquery though. If it's 105 degrees out and you are doing 20 MPH for hours, that may not be considered resonable.
Have you done a stage 1? You can get by with a lot more motor heating situations if your motor runs cooler than stock.
The short message here is you didn't give enough information for anyone to give you an answer that has any value to it.
