Sometimes it’s just not your day. When is it OK to pull the plug on a long run?
If you have an acute running-related pain that’s getting worse as you run, stop. If you started the run with slight symptoms of a cold or illness and they’re getting worse as you run, stop. (Easier said than done, of course, if these things happen when you’re 5 miles from home. It’s not against the law to ask to use someone’s phone and call home for emergency taxi service.)
Other cases are more of a judgment call. If by “not going well” you mean you just don’t feel like it, but you’re fine physically, you should probably see it through. One of the benefits of long runs is learning to persevere mentally. Cutting a long run short because of a mental lapse will make it easier to make the same decision the next time it happens.
Get through tough mental patches by breaking the run into smaller segments. Instead of obsessing over the fact that you still have an hour to go, concentrate on the next 10 minutes, then the 10 minutes after that. If you’re running with a friend, get her talking. Ask about something you know will set her off; by the end of the anecdote, you’ll be that much closer to home.
If your form is starting to deteriorate, it’s OK to stop and collect yourself. Gently stretch areas that are tightening. Then ease back into the run. Sometimes the solution is to run a little faster, because that can get you running with better form. Try picking up the pace for a minute, then back off for a few minutes, then pick it up again for a minute.
If you started the run really tired and things haven’t improved in the first hour, stick it out in most cases, but be aware that you’re probably going to need some solid recovery time later that day. If things are such in the rest of your life that you need to be “on” for much of the rest of the day, then this might be a good day to cut your losses and try for long-run success another day.
If you’ve bonked and have to slow significantly the last few miles, finish it up, and resolve to start the next run better-fueled and at a more manageable pace.