When I’m trying to bust through that dreaded out-of-shape phase, I also (See my incline and intervals post for other workouts that help you get in shape faster) do 30-20-10s. That stands for 30 sec normal speed-20 sec fast-10 sec at peak speed.
It’s the same premise as intervals. And it works your heart real good (I think) under the same premise. But it’s more levels. The big thing about this workout is aside from helping you get fit faster–it’ll help you run faster! Serious. It has helped me get personal records and also increase my endurance.

Here’s how it works:
I’ll go backwards in the description (and sometimes I do run it in backwards order and do 10-20-30). The 10 seconds is supposed to be literally the fastest speed you can possibly do. Like, the speed you can barely get to in the first place. A speed so fast you’re nearly falling off the treadmill. It’s only for 10 sec, so the thought is, you can do ANYTHING for just 10 seconds. And I find that’s true.
The 20 seconds is your fast speed. The high part of your low-high intervals. A fast clip. It’s a challenge, but not the TOP speed you can go. I try to split the difference between the speed of my 30s and my top, top speed that I do on the 10s.
30 seconds is faster than comfort pace. It’s not your warm up pace, but not too far above it.
Some hardcore people just do the 30-20-10s and repeat 30-20-10s for the time/distance of their workout. I like to put a slow rest period after that 10 sec. So I do 45 Sec to 60 sec of warm up or slower depending on my fitness level at the time.

And as with regular intervals, you can change the rest period, and all 3 speeds. Which I do. I try to shorten/eliminate the rest, and I try to push that top speed up. Which will happen naturally as you practice and get in better shape. The 10 sec should ALWAYS be your very fastest speed that you can barely do.

An example workout with speeds:
30 sec at 7 (my warm up speed is 6)
20 sec at 8.3 (to kind make a halfway point between my 30 and 10)
10 sec at 9.5 on the treadmill
45 sec of rest at 5.5 (more time or slower if I’m out of shape, less time, faster speed, or eliminate the rest when I’m in good condition).

Again, this example is for when I’m in good condition. Right now, while I’m dealing with coldness/holiday out of shape I would do:
10 sec at 8 or 8.5 (depending how cold it is in my house–DON’T pull a muscle, in cold go slower!)
20 sec at 7
30 sec at 6
rest at 5 for 1 min
Good luck, I hope the 30-20-10s help you as they have helped me.
P.S. per the usual, consult experts if you have health conditions.
Catty Remarks