nervous

I'm getting nervous about whether or not I am truly at the right fitness level to be aiming for boston - or if I'm overtraining. I'm able to do the workouts at the paces the mcmillian calculator and first program say i should... so far anyway. The thing is - i've found over the last few weeks that i am capable of way more than i thought i was. Track workouts for example. If you would have told me i could do 6 800's at a consistant 3:15 - I would have told you were crazy. I was happy with doing 3:30's... so now i start to think - woah - maybe i am capable of way more than i think - and that the only thing that is holding me back is me. the part of me that is afraid to push to hard - don't push a race too hard - or a pace too hard - b/c you might blow up. Well if i don't try it - how will i know?
So tonight - i'm doing 13 miles at planned marathon pace plus 15 seconds - or an 8:35 - I've never done 13 miles at that pace. but if i can't do that - than i sure as s$#t can't do 26.2 at 8:20... so i'm getting nervous - which iknow will not help. Also on my plate is a 10k this weekend. I'm replacing the dreaded tempo run this week with the 10k race. According to mr mcmillian - my 10k equivalent performance should be a 46:27 - 7:28 pace. I haven't run a 10k since my goal was to break 10 minute miles... so I have no idea what i'm capable of. Last 5 mile i ran was a few months ago and it was 38 minutes -- so if that is any indicator - i should be able to pull a 46 pretty easily... But here's my problem - i am going to over think this from now until the 2 days after the race. And that will probably cause me to work myself up into a tizzy and therefore not do well. I really want these runs to go well to give myself and indicator that i am in the right shape to be training for a bq. And i know that a bad race or a bad run doesn't mean that i'm not there - but i know it sure will set me back mentally. So - wish me good luck and positive thoughts.