I know there are many coaches that love 20-minute steady state efforts at lactate threshold. Some cycling coaches will work toward 2 x 20 minute threshold efforts as the gold standard.
I have always preferred broken efforts or intervals. An example for cycling and running is 4 x 6 minutes at threshold heart rate, power or speed. Recovery intervals are 2 minutes long. A second example is one effort at 20 minutes with 5 minutes of recovery.
That effort is immediately followed by 5 x 4 minutes with 1-minute recovery intervals. I've found the average power output or speed is much better with the final effort being intervals rather than a second steady 20-minute effort.
One study supports my personal preference, showing runners using steady efforts improved running speed by 7 percent and those using intervals improved by 9 percent. It is important to note that both groups improved running speed.
Interestingly, VO2max improved by 10 percent in the steady effort group and only 6 percent in the interval group. In this particular study, improvements in VO2max did not correlate to improvements in running speed.