I’ve decided to do that two challenges as one. So I’m doing my pushups, and immediately after that a set of situps, then back to a set of pushups, and so on until I’m done with 5 sets of each. Burns a bit more, and instead of taking 15 minutes to complete it, it only takes 8 
Deciding to only start at 70% of my max was a good idea, as I can feel the exercises working without killing myself. It also helps with your state of mind as you feel now that you’re doing good and progressing.
How’s it going with this? I should be in my last week now but I fell behind and convincing myself to catch up is proving a tad more difficult than I’d expected
It’s going a lot easier than I thought it would. And what’s best, you go through it a lot quicker, so you don’t have this nagging thought in your mind that you’ve got to go and do 15-20 minutes of exercises. Takes about 6 minutes now to do both challenges!
I’m in my second week now.