What if we can’t join the Mahdi?

Selam aleykum my brothers and sisters.

There’s something that’s been weighing on my mind for years.

First of, do y’all know this hadith?:

“When you see the black banners emerging from the direction of Khorasan, then go to them, even crawling on the snow, for among them is the caliph of God, the Mahdi.”

My question is:

If Mahdi comes how will all millions of muslims reach him? Imagine you live in the west or europe, it would take months to reach them. I imagine there must be no flights but even if how to gather 2 billion people in one place?

Do any of you have an understanding of how this could be possible? Will we be less in numbers? What happens to those who can’t join?

I know the possibility of us being affected by this is probably 0 (Allah knows best) but seeing the direction the world is heading towards, I’m thinking what if it isn’t hundreds of years later? What if it happens next week? How will I join them? What happens if I can’t?

What is your thoughts on this? Sorry if this is a ridiculous or useless question. I’ve been curious for years now and would love to hear from others how they imagine it.