In this thread in the Bug Reporting section, a handful of users were lamenting the lack of a working “Walk” button in Black Mesa. Myself and a few others had a crack at the problem, and found that you can simulate a hold-and-release walk button with the following script snippets:
alias +walk "sv_maxspeed 65"
alias -walk "sv_maxspeed 320"
bind "ALT" "+walk"
Open up your dev console (with the “~” key; you may have to enable the developer console in the “Advanced” section of the Keyboard settings menu) and enter these three lines. Now, when you hold down “Alt”, you will move at walking speed. You may want to play with the numeric values to find speeds you like; another user reported that their preferred values were 80 for walking and 300 for running.
Happy strolling!