i have a serious problem keeping a steady pace. one second i would be doing 17-18 next second i'm doing 20-21. now i have done 20 miles before doing nothing but 19-20, but for some reasoni can't do it anymore
Is this on level ground? Just concentrate on keeping the same effort on the pedals. If you have an HRM, you can use that, just be aware that HR lags effort, so it wont show short term variations in effort.
Riding in a paceline is a good way to learn how to maintain a steady effort.