As paddy448 wrote, Interval Training is likely your most efficient method:

Interval training can cut exercise hours sharply
"High-intensity interval training is twice as effective as normal exercise," said Jan Helgerud, an exercise expert at the Norwegian University of Science and Technology. "This is like finding a new pill that works twice as well ... we should immediately throw out the old way of exercising."

Intense interval training means working very hard for a few minutes, with rest periods in between sets. Experts have mostly tested people running or biking, but other sports like rowing or swimming should also work.

Helgerud recommends people try four sessions lasting four minutes each, with three minutes of recovery time in between.


The new exercise HIT: do less
HIT means doing a number of short bursts of intense exercise with short recovery breaks in between. The authors have already shown with young healthy college students that this produces the same physical benefits as conventional long duration endurance training despite taking much less time (and amazingly, actually doing less exercise!)
The key is a VERY INTENSE, near all-out, for the active period.

Lately, I have heard that 30 seconds on, 30 off, for as little as 10 minutes can be worth 1 1/2 hours of moderate exercise of the same type.

If the intense period is 'easy' you probably aren't doing it correctly.

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HerbM