If you do it once or twice during an 18 week plan, I'd say there's nothing to worry about. It's not something that I'd recommend doing often, though, because to me, the toughest part of a marathon is fighting the urge to stop for a short walk break. So for building mental fortitude, doing uninterrupted LRs is important to me.

If there is no other choice, split it but it is not recommended as a long run.  But when we have no time and it does not happen often, we do what we need to do.

As a general rule, splitting runs is fine.  Indeed, there is one argument that you're better off splitting a lot of your runs because your body adapts not just to how long you stress it but to how frequently as well.  Regularly running doubles forces your body to adapt faster and, believe it or not, reduces the risk of injury.

Having said that, the long run is the one run when splitting it most assuredly does not provide you with the same benefits.  In order to get the benefits of a long run such as increased capillarization of your muscle tissue, you need to run for a minimum of 90 minutes if you're a newer runner and 120 minutes if you have been running for a while.  Of course, that's the "perfect world" scenario and we don't live in a perfect world.  Sometimes, you just do what you can and go on.

 

I agree with this. How much time do you have in the morning? Instead of 9 and 9 could you do one 12? 15? Or move it to Saturday. I've never had a marathon training cycle where my schedule hasn't had to be altered once in a while due to real life getting in the way.

If Sunday looks busy or maybe even bad weatherwise, I do it on Saturday or vice versa when I have flexibility. Last week I had 17 scheduled, and the weekend was tight so I took Thursday off and ran it then. I have also shortened my taper by a week if I bonked on a long run at the end, or was sick.

I ran 18.6 yesterday, and doing it all at once was such a confidence boost which I wouldn't have got splitting it up.

Fuzzy: Great idea.  My problem is that I get OCD when it comes to training plans. Life does get in the way and I have to be flexible.  My plan for the week looks like this:

 

Mon: rest

Tues: 10 mile tempo w 5 @ 7:20

Wed: 4 miles recovery

Th: 8 miles 8:30 pace

Fri: rest

Sat: 7 with 8x100m strides

Sun: 18 @8:50ish pace

 

I suppose I could switch Saturday and Sunday and make Sunday a recovery run?

Definitely! I've often done that myself.

Switching the long run to Saturday is fine. If you do, make sure Thursday is an easy run since you'll have less recovery before your long run. Sunday be ready to modify mid-run if your legs are too tired. From the schedule it looks like you normally have a rest day after your long run and you wouldn't if you simply swapped Saturday and Sunday. You want to keep Monday as a rest day leading into Tuesday's quality workout.

YES!! Thanks, wc. I will keep the mileage the same on Thursday but I will take it easy and not push it. FYI, I am on the Pfitz 18/55 plan.

Jack, you and I are in a very similar place in our training and have a similar problem.

I am in week 9 of the Pfitz 18/55 training for the Philly full on 11/17.  This whole plan I have reversed the Sat/Sun runs as I reserve Sunday for church and family time in the morning.  Thus Sat is better for the long run, plus I don't have to get up so early to complete it.  I treat Sun as a recovery run, sometimes forgoing the strides and just running easy.  I have had no noticeable impact on fitness or the ability to hit my Tue/Thur quality workouts.  I love the Mon/Fri rest days as then I do not have to get up as early.

Don't sweat it, looking forward to your 3:30 full marathon race report in December.