Oh yes, Morgan = Janice. Oh. My. God!
They travelled 500 miles by car from the hospital to Noah's old home. While they were there, they decided to continue to Washington and I think they said they were only 100 miles away. During that 100 mile journey, their cars died and they ended up walking and were found by Aaron a few hours' drive from Alexandria.
Morgan can't be following them directly, as Rick's group drove most of the way, but he found the map to Washington (with a message from Abraham addressed to Rick Grimes written on it) so he knows that Rick is probably heading to Washington. I assume he'll catch up with them at some stage because why else would they show him (apart from "Oh. My. God!"), but I'm fairly meh about it.
I don't think they need to show the beginning of the outbreak, or how it came to infect every person, or that there will necessarily be an end. It's just the world that they are living in.
I find it hard to believe that the people of Alexandria can be so sheltered for so long. I know Deanna said that Washington DC was evacuated, but surely there are more survivors and zombies wandering around? Or other groups of survivors who had the same idea as Rick to head to Washington. Rick's group were in rural Georgia and there were zombies everywhere.