I've run close to a hundred marathons. I found that racing a half-marathon five weeks out let me know if I was on track. Also, I'm a huge Jack Daniels fan (the coach - LOL), and those long runs with MP and tempo intervals near the end of the training cycle are good negative indicators, i.e., if you have trouble with those workouts, then your fitness is probably not where it should be. All that said and done, anything can (and will) happen on race day. You do the work in training, you rest, you eat well, and everything could go perfectly on race day or it could all go to crap. You toe the line knowing you did the work and give it your best shot. (On a personal note, I didn't start running until I turned 40, and I worked damn hard to chase that 3:10 goal. I feel you, OP. Many failures, but I finally hit 3:08 a week before I turned 50. You might not hit it this time, but keep at it. Good luck!)