I don't mean to keep touting high intensity intervals as gospel.  Yes, they're effective - very effective.  The downside is:  they're not necessarily enjoyable, which is putting it mildly.  Let's put it frankly:  they're hard.  Perhaps short interval training needs better a better PR image.  Another term for them is Low Volume Interval Training - let's say LoVIT for short.  That's more catchy. 

• 10 min W/U
• 20 mim L3
• 5x1/2 Lap (1:05 min) @ L6
• 20 min L3/L4
• 3x1/4 lap Seated Max