From the Hammersmith & City, Circle Line and Metropolitan line trains at Kings Cross St Pancras, there are three staircases leading away from the platform, going to two distinct exits.
You can see how this works on this 3D map of Kings Cross St Pancras underground station
For an exit to Euston road going westbound, your best bet is to exit via the western two staircases, which head to common ticket barrier. These barriers are only used by H&C,C,M trains, so are rarely very busy. Even with a train arriving, it only takes a few moments to exit at rush hour.
Avoid the interchange staircase, which is the far eastern one. That is a longer walk to the ticket barriers, and is shared with traffic changing to other lines, and the barriers are shared with several other lines. Consequently it's more walking, and more people when you get there
When you leave the gateline, you have two choices. One it to turn left, go past the ticket office (with often queues and lost looking tourists), up the stairs towards St Pancras station, then left to exit onto Euston Road. This is the most direct, but the busier. It may be easier, as the exit ticket barriers are on the left.
Otherwise, exit to the right, and take a right just after the ticket machines. This goes up a fairly quiet staircase, then at the top go right and right. This takes you out onto Euston Road. Almost never anyone on the staircases or exit, but you do have to cross the flow going into the underground at the ticket barrier twice.
