I do this every day except Sunday. I started last summer using half pushups and half planks instead of the full ones, and struggled even then, but now I'm strong enough to do all full ones with ease (usually). Its been fun to see muscle development take place!
Once this routine has become easier, do 150% of everything, (15 pushups, 30 lunges, 1.5 minute plank, 30 squats, 30 mountain climbers, and 2.25 minute wall sit). I have also added a 1-minute superman, just for giggles and grins :)
So do your best, and if this is too hard, take it down a notch and work your way harder! If it is/gets too easy, just add more rounds or burpees or something.