Pyramid:
1:00 on - 0:20 easy
1:00r
2:00 on - 0:30 easy
1:00r
3:00 on - 1:00 easy
1:00r
4:00 on - 2:00 easy
1:00r
5:00 on - 3:00 easy
1:00r
4:00 on - 2:00 easy
1:00r
3:00 on - 1:00 easy
1:00r
2:00 on - 0:30 easy
1:00r
1:00 on - 0:20 easy
1:00r

Aim is to hold steady heart rate, just below high intensity.

Split on the opposite sides of the pyramid to match.