Short answer: My schedule.
Long answer: The threshold tends to be more about 'how much time I have' instead of 'how far am I going to go'. I typically run 2 miles each morning before work (Tues - Fri). I then do a longer distance for the weekend. I used to run both Sat an Sun, but now since I run a local runner club (LIRRC) 5k every Wed, I take Sunday off (as well as Monday).
What I'm trying to say is: For me, I decide how much time I have, and then decide how I can use it. Running 1 mile each morning was good until I realized that the time it was taking me to get ready and to go was a poor ratio compared with how much time I spent running. So, I bumped it up to 2 miles each morning and now feel that the distance better matches the time spent preparing.
Hitting a wall? I haven't had that happen yet. Well, not while running. I've always felt that I can keep going. Sometimes, during a race, I'll feel 'spent'. As if I just don't have more energy to go faster, but I haven't yet felt that I couldn't go further. Having said that, the furthest I've run was 13.1 miles. I don't know if I'd feel that I couldn't go any further if I was running a full marathon.
I think that in the past reducing my weekly mileage (from around 17 MPW) to my current weekly mileage (around 15 MPW) might have had a bigger mental impact. But, I made the decision that with all that I have going on this Summer, I don't want to run on Sundays. Having a slight knee problem (which didn't seem so slight at the time) gave me a good reason to look to doing something else other than running. So now I bike on Sundays if I have the time. That way, I don't feel guilty about not running, I 'cross train' (which helps reduce the possibility of injury), and I have a different activity that puts me with a different group of people (typically I run solo, so riding with a buddy has a very different appeal). I am trying to look at things with a positive attitude and for now it is working.
Cliff Notes on what has worked for me:
Decide how much time can be spent for exercise
Use that time as effectively as possible (run from my house whenever possible (IE: no driving someplace to run))
Adjust things as necessary (distance, time spent, which exercise, etc...)
Remember to mix things up a bit every now and then (the same routine over and over eventually led to injury and the need to add in a different sport)