Sunday Morning Runs - how far is too far?

Regular readers should remember that I'm spending my Sunday mornings running and Soo's spending her Sunday mornings coming to pick me up from somewhere way out East. There runs started off well, but now I've got a problem.

I started off running between 10 and 17 miles, depending on how fit I was feeling. For the last few weeks, I've been running from home to Shoreham. This is a 21 mile route that takes me around three hours. The first time I ran this route, I had to stop before the end and walk most of the last mile as my legs had stopped working. All the muscles in both my legs were extremely sore. I'm hoping that this is the famously painful "wall" that marathon runners talk about - if it's not, then I don't think I can handle the real "wall" when it comes.

Anyway, the last couple of times I've run this route I've still been in pain and quite tired by the end, but I've managed to keep running for the whole distance and I know I can run further. And this is my problem - should I extend my training runs to 26 miles? All the training plans I can find say that I shouldn't - 21 miles is plenty. But I can't really see the logic behind this. My body has responded well to the 21 mile runs, so why shouldn't it respond in the same way to 26 miles? And, anyway, I'm not following a training plan. I'm just running a few times a week when I have time.

We'll see how well I'm feeling next week before I decide how far to go. Who knows? I might end up in Brighton again!