Advantages/Disadvantages of Looped Training Course

Advantages:

1. As mentioned gauge progress

2. Use your vehicle or some other method as a pit stop for nutrition and hydration needs, which is similar to race conditions.

Disadvantages:

1. Your legs will be trained for that course, meaning the characteristics of your course...small hill 2 miles in, big hill 3 miles in, flat for 2 mile, repeat.  You could run it back wards to combat this, but should try to mix it up a little.

Disadvantages: 

The race you are training for is a different terrain.  Ex: you are training on a flat loop and the marathon race has rolling hills.

If you are running on the same side of the road constantly on the same course, due to the pitch in the road (for rain runoff) your legs are hitting at different distances and this may cause runners knee ...which is exactly what I have right now!  During my Long Run this weekend I had to switch to the other side of the road to somewhat alleviate the pain.

When you run the same course a lot, your body adapts to the terrain physically and mentally -- meaning mentally: you know when to push, when to coast ; physically: your muscles get used to running certain flats for certain distances and running only the same hills.  This is bad as you become comfortable and possibly stagnate.  I've read about this numerous times in my Runners World and Triathlete magazines, and what happens is you reach a plateau and get comfortable so there is no progression.  Will all of this affect you for one single season of marathon training -- probably not .  If you run the same loop for several years, it may become a problem. 

Advantages:

You know the loop so chances are it's a bit safer.  Drivers get used to you and know to watch out. 

Mentally it can be easier.  Especially when you're doing a 20-miler, and at mile 15 you can say to yourself, "Ok, just one more loop to go." 

You're less likely to go down a route that has killer pittbulls.  Serioulsy, I hate running down a new road -- I'm always waiting for a dog to come running out and bite me.

Suggestions:

Personally I've been running the same route for three years - a 1.5 mile loop and a 10 mile loop.  To mix it up run the loop counter clockwise some days and clockwise other days.  One way is probably easier, so run that way on recovery days. 

Do HILL REPEATS.  Find the steepest hill on your route.  Run up it at a Tempo Pace, turn around and run down easy and Recover, then turn around and run up it again - based off your ability or running plan, do this as many times as possible without risking injury ; and try to do it once a week.  Even if your marathon is flat, running hills will help greatly ; and if your marathon has hills, doing hill repeats will help you run the hills on Race Day with ease and allow you to maintain a consistent Marathon Pace.

Again, I've been running the same 1.5 mile loop for 3 years, and doing my Long Runs on the same 10 mile loop, but just this week I ended up with a sore knee for the very first time.  Perhaps it's a sign?  Perhaps it's cause I wore my beat-up old trainers, the bottom-line I think is you can run the same loop - just don't run it the same way.  Mix it up and find variations and eventually find another loop and alternate the two. 

Good Luck at your Marathon!

My 4 19+ runs this year, I did same loop with variations. I drink a lot and need refill so I would run a loop with bathroom and water at 3.1 miles then stop back at home at 5.5 miles. For me it was mentally good to know I would practice my nutrition/hydration right.

Boring? Maybe but also more comfortable.

From my house there are so many ways to run and I only go 2 of the 4 directions as there is a highway one direction from my house which would make the loop to long without more water.

I agree, every run / ride turns into a time trial.

Also, if I'm doing a 20 mile run I want to be 5 miles from home when I hit mile 15.  A looped course would be too easy to cut short if it starts to get too hot/cold/rain/snow/wind....