Just be careful. If a quest giver gives you a time limit, like a letter saying “get here in one month”, they mean it. It’s possible to silently fail the main quest this way, but it’s also possible to get back into the main quest line via some other quest paths into it.

It’s all fairly complicated. Thankfully failing doesn’t stop you from exploring the world, interacting with NPCs, or doing other quests. You can always use the console to restart the main quest line quests when you’re ready.