Tour de Houston is a great ride, but will be crazy busy at the start. It's a huge crowd. You should register online and go get your packet at city hall ahead of time, don't even think about trying to fight the lines the day of. IIRC, it's one of the few rides that requires an ID to get your packet so you can't ask someone else to pick it up for you.

Parking downtown will be hard with so many people coming in, so allow extra time for that too.