Still doing my planks on my knees. Made it to 1 full minute on knees with straight arms!

Knees and forearms is a whole lot harder: 40 seconds is my max.

And when I occasionally TRY a toes and forearms version, the best I can achieve is still under 15 seconds.

I've been going back and forth between the two easier varieties and trying to do planks twice a day - when I first get up in the morning and right before going to bed.

I do one, rest awhile and repeat, rest, repeat, etc until I just CAN'T bring myself to get into plank position AGAIN. Rather than trying to hold for a preset period, I just try to hold each plank for as long as I can. Soon as I feel my back start to sag and I can't pull it back up, I drop and rest. Figure it's better NOT to hold a saggy plank and end up damaging my back.

I usually manage three to five reps but am only able to hold each plank for a much shorter period than the previous one. When I'm done, it's usually all I can do to get myself back up off the floor. Takes less than 5 minutes all totaled.