@wilkenl Yay for core exercises. One point though. I think the practice of doing longer and longer planks is not the best way to add challenge. Stuart McGill my favorite expert on low back and core matters suggests holding planks 5-10 seconds. No more is needed or to be advised. He suggests other ways to add progressions to core exercises than length of time holding. I think the logic is that anymore time than that and you are interfering with the bodies ability to clear waste products from the muscle tissue and not helping improve muscle strength further. Sorry I don't have time right now to find what he stated as the reason behind it. But I am sure his suggested timing is 5-10 secs.
I have been following his advice in this area for 8 years now and have greatly improved my low back - hip issues and core fitness. His book Back Mechanic is great for the "layman" and there are tons of free interviews with him on Youtube.
Also 5-10 sec holds are just way more fun.