Today's challenge is going to be as much a mental challenge as it is physical. So spend some time getting your head in the game before you begin.
Hold forearm plank for time.
Rest 90 sec.
Hold tall plank, for time.
Max forearm plank, 90 sec rest, Max tall plank.
Focus on quality, over time even though today's challenge is for max times.
Before beginning, know what your previous best times are for each of these. There will be mental battles before today's challenge ends and knowing your target times while in the plank will help you stay in right positive frame of thought when your minds telling your body it's time to quit.
When you are at your limit, you should remind yourself that it's going to take all this time (and suffering) again the next attempt, just to get to this testing point. So don't waste this opportunity. 20 sec after you stop, and the pain fades, do not be regretful that you didn't fight for a little longer.
Quality or nothing! The time is meaningless if it's not a quality hold.
PS: Feel free to take as long as you need between planks. This is your challenge. Your journey. But if you take more than 90 seconds, you can not relate your challenge to when it is done as laid out above.