Finishing the quest lines isn’t easy, but I don’t think they’ve got a time limit.
The locations I was directed to were quite distant, though, so getting there without a vehicle capable of smashing through zombie hordes clogging up streets in the many towns you can’t skirt around on the way to the targets is a deterrent in itself.
In general, if a quest seems to be too difficult for your character it probably is. Take your time. In some cases it might be possible to “probe” a quest and back off if it seems too hard.
One of the quest lines end with a vaguely quest like mentioning that’s supposedly related to some end gamish level thing you may encounter at some time (much) later.
I can’t really understand how you’d be able to even perform the quests performed in 5 days, as the two locations are 1-3 days away by foot in different directions (assuming you knew in which direction to walk and where the bridges crossing rivers are), and their targets are similar distances away (in my game).
I waited quite a long time with the quests involving attacking and killing, and think that was a good choice.
Any quest involving going to an underground location infested with monsters is dangerous, but can be handled with training, equipment, and luck (there’s always the risk of descending down stairs and immediately get grappled and pounded to death before you can try to retreat).