15 minute HIIT (Example, there are variations)
1 minutes steady --- pretty good pace
:30 bust your a$$ --- go hard for 30 seconds
:30 steady --- slow down a little slower than your original steady
:30 bust it
:30 steady
I would do 12 "sprints" then cool down a minute or two.
Should get your sweating pretty good.
Bust it means over at least 100 rpm on the bike and keep it there. Alter resistance as you need to.
If you can do :30 pretty easily switch is to :45 or 1:00 for each steady/go hard. 15-20 minutes is the best range. You shouldn't be able to physically do more than 20 minutes if you're doing it with good intensity.

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