I've always answered "no" to this question, even though I personally feel like I take my running pretty seriously. I'm not fast and I've never competed and won anything, but I've finished a whole bunch of races, mostly 2nd quartile stuff, ahead of middle-of-the-pack. Boston certainly looks like a remote scenario, if I had to qualify to get in (how in the world do you trim 65 mins off over 26 miles?).
So why do I still feel like I'm a serious runner inside? Mainly because I run something, pretty much every week. Except for Decembers. With holidays and the weather, I gave up trying to stay on record and just decided to take the whole month off. Other than one or two bouts of the flu, I've run every week since I started in the summer of 2002, at least once in order to count, on average 3 times/week.
Looking at some running blogs out there (start with Alison's and follow the links), there's some serious miles being logged. More power to them. This blog will undoubtedly be different. Just like my races, it'll be a little slower, a few notches below the first wave of runners. Respectable I hope, but nothing to call home about. BTW - for some perspectives from a broader range of runners, try the Runner's World Forum - something for everyone there.