The recovery period between intervals is an extremely important part of the workout. It allows us to focus on pushing that race pace without overstressing the body. By breaking up that workload we are able to spend more time in that Level 4 zone without the lingering effects of a full length race effort.
The 120 bpm mark is set to be solidly in Level 1 territory for athletes of all abilities and is used as a mark to show that full recovery is attained between interval sets. One sign of potential overtraining is an inability to recover between sets (an elevated heart rate for many minutes longer than expected). That being said, the 120 bpm mark is arbitrary and not specific to you. We would expect that you take at least 2 min to recover between each interval or until your heart rate is back to Level 1.