"Brick" is in this case obviously a metaphorical term, and can be construed as an edifice -- you build the wall laying one brick upon another -- or, if more appropriate, as that thing each of your legs feel like once off your bike and afoot.
Keeping with masonic terms, perhaps a closer analogy is a sack of cement. That is the substance you'll swear is coursing through your veins as your cycling muscles are now asked to go back and forth and up and down instead of around in circles. Not to worry. There's a 50/50 chance that sensation will abate within the first two miles of running.