Lots of good responses here already but just to add an idea: I like to do my tempo run on Friday and then my long run on Saturday - especially when marathon training. That way I start my long run with tired legs from the tempo run. It trains them to work hard and long even when tired. If I do that, I usually only go 20 but sometimes will go 22.
That seems like asking for injury. There is a reason almost any training plan, Daniels, Pfitzinger, Hansons, etc., put hard workouts (whether that be intervals, tempo runs, MP runs, or long runs) with several day of easy running in between.
I can see that. I don’t do it absolutely every week and one has to have a solid base to do it but it works for me. It may not work for others. I’m trying to remember where I got the idea from as I know I didn’t create it on my own. I read it from some article somewhere but it is escaping me right now. Regardless, it is an approach.