I like to build strength and anaerobic endurance on the bike by doing hill repeats in a gear that I have a hard time turning over by the end of a 1 km hill. I'm 195 lbs and find 53/21 is usually the right gear for the job, YMMV of course.

I wind up to 35-40 kph on the approach and try to keep the gear turning over as long as I can while staying seated. Once the real cadence is done I grind it out as long as possible while staying seated and transition to standing once I can't grind it out in the saddle any longer.

Turn around at the top, coast down and spin easy for 5-10 minutes and then repeat. I do 2-3 one day a week in the early season and progress to 5 or 6 as strength and recovery allows. When it gets easy go up a gear and keep at it.