A 63 watt average must have been taking into account all the time you weren't pedalling, that's extremely low - you would struggle to ride into a slight breeze.
It's certainly possible for a leisure ride - 100W give you ballpark 20-25km/hour. The drag scales quadratic, so if you go below 20km/h, you'll end up somewhere in the region of 63W average. Peak output would still exceed 100W.
