Everyone seems to think
a) there is such thing as going to too slow in miles 1-2 AND
b) you can't take advantage of the downhill final 10k because your legs will be trashed
but in reality if you
a) take the first 2 REALLY easy THEN
b) your legs won't be trashed for the final 10k
No one is forcing you to fly downhill at the start... you can head to the side and hold back... that's what I did and I negative split an 8 minute PR
I will say it again -- nobody has ever finished Boston & has thought they didn't hit Hopkinton & Ashland fast enough. I would even extend taking it easy to 10k. This is pretty much the recipe for any marathon, unless you have a 40 degree day on a pancake flat course & you're gonna lock in from the gun. Boston is a racers course. Relax early and you'll be able to run marathon effort in the hills and then can take advantage of the finishing miles. But this still more/less results in running pretty even in the 1st half & 2nd half. It makes sense to see a small positive or negative split at Boston. If you're not close then you ran too hard too early or didn't factor in an adjustment for weather (Monday should have been ~2% or so).
If you wanted 6-flat/mile on Monday, your real target should have been ~6:10/mile, adjusting for weather/course. So taking it easy early on would mean 6:15-6:20 in the 1st 5k, if not 1st 10k. Then 10k-25k you should be locked into 6:10. 1:21-low/mid front half. 15-16 should be your fastest mile of the day but should feel under control. Let your heart rate come down a touch. Continue running marathon effort through the hills. Lose :10-:20/mile with purpose. You shouldn't be hammering to try to keep pace. Run marathon pace from 18-19. Get to the top of 21 & it's go time. 21-22 should be as fast as 15-16. That's how you know you're ready to cook through the closing miles. Ride the downhills & you should probably be hitting splits at or faster than your goal target (6:00-6:10/mile off of the adjusted 6:10). Finish in 2:41-high/2:42/low. Worth your 2:37-38 on the day.