sub-3 or blow-up trying

So...I'm attempting my first marathon "training run" ever.  I think I've done 6-7 marathons and always with a taper in the "race mode".  Hartford (40 minutes away) Marathon falls on a Saturday and my wife's running the 1/2, so thought I'd use this as an experiment in preparation for my race marathon in November (Philly).  My new shoes came in earlier this week and they are tremendously easier on my achilles- I think my achilles issues this summer are a shoe issue.  It's been tough over the past year or two figuring out my shoe situation.  Every time a shoe begins to get comfortable, the model gets retired.  I haven't been happy with trainers for a while (since the Saucony Trigons disappeared) and now my race shoes have been discontinued (Saucony Tangents).  Ordered a couple pairs and the ones I think I'll go with for Saturday are a bit heavier (10 ounce) than I've used for marathon, but being a training run I want a little more support- Asic ds-trainers. 

Anyway, wanted to get a short mp run in before Saturday.  Went to the track and did a 1-mile warm-up and 3 mp miles- 6:50,6:47,6:21.  The last one was firm, but felt relatively controlled.  I'm not sure if the shoes are fully broken in for Saturday, yet they felt good.  Now, I'll take a couple days off and hope for the best.  My only goal for Saturday is to get a decent slow mp (7-7:30) for 20miles and anything faster or beyond 20-miles in that mode is bonus.  If I start to hurt (achilles), I'll be more than willing to walk/run the 2nd half, so as not to do any damage, but I think I'm turning a corner.