You will clear customs before you leave YYZ, so DFW will be an easy transit.

Skylink runs a loop around DFW, makes several stops at each terminal and each terminal at DFW is linear. The trains are fairly fast and frequent.

If you have a long layover, and no lounge access, head over to terminal D, which I think is the nicest terminal with the widest selection of things to do.

It should be pretty painless. Do double check your gate information in DFW, some days I have gotten a lot of exercise going from A to C and back to A as gates changed.