What kind of advice would you give someone starting out (like me), who wants to build up distance? I feel like I keep hitting a wall around 4 miles, where my legs just want to stop. Am I running too hard? Should I ignore my time and just run super slow?
Exactly. All runners who are starting out should not run for distance or speed, they should run for time (I know, you'll never read this anywhere, but it's in all the books . . . between the lines). The reason is that you are initially looking for physiological adaptations to occur - both in the way your muscles, bones and joints handle the movements and also how your body handles the overall exertion.
So instead of saying I'm going to run for 4 miles, just run for 45 minutes at a comfortable pace. If you have some spare funds laying around, the best thing you can do for your training is to get a heart rate monitor and use it make absolutely sure you are running as slow as you need to (be prepared for how slow you will need to run to stay in the aerobic training zones at first - it's ridiculous!)
Start out easy with 2 or 3 easy, shorter runs during the week and one longer run on the weekend. At first you don't need to worry about speed, but if you want to add some speed work - basic sprints and strides and whatnot, tag it onto the end of a run or two during the week.
Depending on how you feel, add time to your weekly plan in small increments (like 5-10% a week). But for the first few weeks you are really just interested in getting used to the motions, the impact on your body, etc.. Don't fall into the trap of adding significant time or distance too soon. You may feel fine now, or even a few weeks from now, but in a couple of months it will backfire on you.
After you've done this consistently for like a while, you can think about adding that distance.
Another idea about adding distance is that you don't need to be afraid to stop running. By that I mean if you plan to run 5 miles, but you know you will feel absolutely dead after 4 miles, maybe stop at miles 2 and 4 and walk for 30 seconds to a minute and catch your breath. See how that works for you - I found early on that it really let me keep moving longer and the drawbacks to doing this are pretty much nonexistent.