thanks all for your replies. I'm definitely feeling better about having to use my mtb bike on the road for the duathalon.

I will have to check into putting slicks on them, I need all the help I can get

Doing the duathalon is mostly just for fun, but honestly I really don't want to finish last and I was just concerned that I'd probably be at a big disadvantage riding a mtb.
Just means Ill have to work harder and push stronger......