Since you pass through Immigration / Customs at YYZ, it's considered a domestic flight, so all you have to do is hustle to D for your connecting flight.

Yes, the connection you want is airside, so no need to exit secure area.

There is an old train in the basement, but if you choose to use that, you exit secure area and IMHO have no chance to connect. Very few people know about this these days BTW.

So just take the ABOVE, DON'T exit secure area, and all should be good.

UAL.com has a map of IAH, and you can look there to see what awaits.