Many HIIT classes are quite intense and extended. But a lot of research shows that brief high intensity sprints ( ie. a set of 6 all out 30 second sprints, each followed with 1 minute moderate spinning) will get you most of the fitness you need. Body by Science says a similar brief group of 5-6 core weight lifting exercises taking muscles to exhaustion in single sets of 8-12 will achieve good muscle training. I use that approach in the winter and when the good weather arrives I substitute the sprints with riding and keep up the weights. If you don't routinely do all out sprints during your rides you could undoubtedly keep doing one or two of the spinning sets with no problem.