math that made me sick

Last Sunday I competed in a 5K race that I finished in approximately 21 minutes. I had a average 6:48 mile time. Today, I did the math to find out the average mile time to finish a marathon in 3hr. The pace - 6:51/mile. I don't know why, but I felt sick as soon as I figured that out. Can I do it? Maybe. I think so. But I need to take my training much more seriously. Well... well.... Hmm. I think that I will look up a training schedule and get busy.
The qualifying time for Boston is 3hr 10min for my age/gender group. Still, that means that I pretty much have to average 7 min/mile for the whole thing. I can't do it now but I can get there.
me