There are many styles of Brick sessions, but I like to keep things simple. My two favourite brick workouts are:
a) Endurance Brick During pre-season or the early stages of a targeted race training plan, I’ll do a session comprised of a longer ride (anywhere between 90min to six hours), immediately followed by a 20-40min run, all done at a lower intensity. I usually do these Endurance Bricks once or twice a week. The purpose is to build aerobic strength and endurance and familiarize your muscles (and mind) with the sensations caused by the transition from cycling to running.
b) Race Simulation Brick As fitness builds and races approach, I introduce Race Simulations into my training program. These sessions begin with a ride (anywhere from 90min to four hours, depending on what races I'm focused on). After a decent warm-up period, a large proportion of the ride is broken up into intervals at targeted race pace/effort level. Then I transition straight into a run off the bike, usually 3-10km (again, the actual distance will depend on what race I am specifically prepping for). This run session can be done whole or broken up into longer, sustained segments or intervals, which is a good idea if your upcoming event is longer. The goal is again to “hit and hold” your goal race pace/effort level.
These Race Simulation brick sessions are challenging but fall within your limits. They combine duration and intensity, like your upcoming race, so they may require more recovery and perhaps a little "mini-taper" in the preceding day or two. It is worth considering your training plan and session scheduling when incorporating these bricks. I usually only perform one of these Race Simulations per week and most often only in the last 4-6 weeks of my build-up.