You're right to be daunted.
You've overcommitted yourself
You are a FT SAHM, you also work one day a week, and you study.
So you are doing an exhausting thing all week (looking after either a baby and a toddler, or two toddlers), then on one of your two more restful days you are doing a full day's paid work. On top of that you have set yourself a goal of getting a qualification and need 25 hours or so of study time, during which you need to be alert and concentrating well to make the best of it.
On top of that, for some unknown reason, you have taken on a pointless hobby commitment of training for a half marathon.
You are already over-committed - why take that on now? It's just putting needless pressure on everything else. You can run for pleasure and fitness without putting a training plan on top of you, and arguably everyone in the family.
I'm a demon for this, but all the "it's doable" misses the point that
a. it's barely doable
b. with so little rest time, you won't have the mental space to do all (or maybe any?) of those things well
c. your life while doing all these "doable" things will be one long drudge - where's the fun time if you do all these hours dong things?
What are your priorities here?
Before scheduling all your time away in excel, sit down with a blank piece of paper and THINK about what matters most to you?
Is it enjoying (as much as possible
) the time with your children?
Well that is in direct conflict with being stressed.
Is it doing well at this course?
That requires more than adequate study time in a decent frame of mind.
What about the job?
Is it leading you somewhere you want to go? Or is it for money? If money, are there economies you could make to enable you to give it up?
The half marathon is just madness as far as I can see. Why voluntarily heap extra pressure on yourself now?
There are no prizes for running yourself ragged.
Be a bit kinder to yourself. If you're gong to do this study as well as looking after two such small children, you're going to need to be.