Do not let the future trouble you. You will come to it (if that is what you must) possessed of the same reason that you apply now to the present.
All things are meshed together, and a sacred bond unites them. Hardly a single thing is alien to the rest: ordered together in their places they together make up the one order of the universe. There is one universe out of all things, one god pervading all things, one substance, one law, one common reason in all intelligent beings, and one truth - if indeed there is also one perfection of all cognate beings sharing in the same reason.
Everything material rapidly disappears in the universal substance; every cause is rapidly taken up into the universal reason; and the memory of everything is rapidly buried in eternity.
For much of my youth, Matthew 6:34 didn't compute for me. "Sufficient unto the day is the evil thereof." I kept thinking about it and then it finally made sense. The events, both planned and unplanned, that happen today is enough for your attention and focus. Don't worry about the past or the future. Today's events are enough (sufficient). Marcus says something similar in the 8th passage of Book 7. No doubt, plan for tomorrow, for next week and beyond, but don't let the future trouble you beyond your plan.
In the second passage, he is saying nothing more or less than: there is a fount to all this we see in the world and universe. Everything is related to everything somehow and in some way. This thought is useful when you encounter something in your life that seems to go against you. If you remember all things are inter-connected and related and that there is a fount or one source, it may help you embrace whatever has come into your life.
In the last passage, Marcus reminds us that any event or anything, will quickly become a part of the universal substance. We are made from stardust and we will return to star dust. Us along with everything else.
(see also Citadel p. 43, 141, 182)