You should be fine. The area where you check in for your flight/go through security is only a short (2 minute) walk from the rental car drop off area.

The only negative is that UA frequently uses the CRJ200 on the BUR to DEN route; this is a pretty lousy plane for a flight of this length, in that it has no first class or E+ and the exit row leg room isn't much better than the regular seats.

Of course all this would be moot if you are late due to traffic!