Even with limited training time, you can increase your cycling threshold speed with this efficient workout.
While you can perform this session either indoors or out, I find that when I'm really pressed for time, indoors is best. I can be on and off the bike in just about 1 hour.
10 min aerobic spin including 5x 30 sec ramp-ups to a hard effort.
5 min easy
20 min total
10x 1 min as hard as you can sustain
Rest Intervals (RI): 1 min
5 min easy spinning
This workout is simple, effective... and can be very challenging!
I always preferred to do this session in the aero position, and focused on engaging my core and generating power from the hips down. It's important to keep your upper body relaxed and still during the hard efforts.
Also pay attention to your cadence, making sure that you're keeping it within the range that you've specified for the day.
Even when you're on a tight schedule, this workout can help continue your progress. Give it a try and let me know how it goes.