If you want my opinion, it is to pay off your mortgage.
It's not sexy, but the idea of interest rates staying low for the next two decades is unbelievable to me.
Of course I and we don't know your full set of circumstances so ultimately you have to make the choice for yourself.
I think though - the reassurance will enable you to sleep at night and keep you on track doing your projects.
Personally I intend to construct my own house - and it is mostly because I want the security, I think it is underrated when the economy is proceeding in a generally positive direction.