Hey @Portobello_T, mcalista nailed it: just make them longer. Once you find a repeatable but high enough intensity that raises your breathing/HR, makes your legs light up a bit toward the end of each repeat and gets a little more demanding with each subsequent repeat, you’re on the right track.
Then you can make them longer (e.g., 30s to 1min to 90s to 2min, etc., all at the same intensity) or make them denser (e.g., 30/30’s, 30/20’s, 30/15’s) or even a bit of both (e.g., longer sets of 30/30’s, back to shorter sets of 30/20’s, then longer sets of 30/20’s, then back to shorter sets of 30/15’s, then longer sets of 30/15’s…you get the idea).
In all cases, you can cram quite a bit of work into an hour. Typically, VO2max work reaches a ‘productivity tipping point’ around 30 minutes of an hour-long workout, but there are always exceptions–riders who can effectively recover from more than 30 minutes even within 60 minutes, riders who can tolerate more than 30 minutes if the VO2max total is distributed over more than an hour-long workout, riders who are so accustomed to 30 minutes of VO2max that more than 30 is necessary to serve as an effective training stimulus.