Very sub-standard core condition

I (name hopefully withheld) have been doing the basement beginners tempo program. My sit-up ability is poor and I have trouble completing the 10 reps.

Should I let one exercise hold me back from moving to the intermediate tempo series?

I am going to try and increase my volume on the high intensity days to do 100 core exercises but I am wondering if it is okay to only get a partial movement of each exercise? What I mean, is it best to increase the volume of whatever pathetic movement I can do and the fuller range will come later…or should I do less volume and more full-range movements?

Any suggestions on increasing my core strength would be appreciated.

Well you want to move the core exercises around as much as possible so you can look at it two ways.
1: If you CAN improve at a reasonable rate in the one area, I’d stay where you are for awhile and let those exercises catch up. You can make them easier by elevating a board slightly and doing abs with your torso elevated a bit to make the abs sort of “downhill” You would like to get the controlled movement in on the abs and this might help.
You can then work on lowering the board till you’re flat on the ground again.
2: If, even with the downhill approach, you CAN’T progress at a reasonable rate compared to all the rest of your exercises, I think I would keep the reps there down and move forward and count on other exercises to bring up the core strength indirectly (exercise anywhere strengthens muscle everywhere via the “crossover effect”).
In any event, I would consider a longer GPP to allow the core to build up by direct and indirect means as core stabilization will be important to keep you in a good sprint posture.