My GPS (Dan's) cut out for a short section and made a straight line of the winding climb off N Leverett Rd, which would account for the slight difference in milage and elevation from Liam's data. I've included Liam's Strava link which should be considered the more accurate of the two. 39.17 miles and 6,120 ft. Elapsed time would not have been affected so I split our 4 second difference for the reported time.
For anyone wanting to attempt this, check out this old guide: https://www.amherstma.gov/DocumentCenter/View/610/Robert-Frost-Trail-Guide . It's still mostly the same except for the re-route near Pulpit Hill, which is well-marked. Almost all of the trail is well-marked and should be ok to follow with attention to blazes. The three of us are locals who have run all over this trail for many years, and between us we know the whole thing, so it might be harder for people who are unfamiliar.
Around miles 30-35, things get a little tricky as the trail gets into more populated areas. The following tips should be helpful:
1) Pay attention and know the route through the suburb neighborhoods on both sides of Rt 9, there are a lot of turns off the roads onto trails, some of them are minimally marked, and blazes are limited along the roads.
2) No blazes on Well 4 Rd in Lawrence Swamp, keep going almost to the building at the end.
3) Coming out of Lawrence Swamp into first field, take a right, away from power line (double blaze visible), then coming out at another field, go left towards power line and go around the right side of the field (no blazes here but there is a path). Looking over the satellite view of the Strava map would be helpful for this part!
4) Cross Bay Rd and stay with the power lines. No blazes here but watch for a well-marked right.
Cheers and happy trails!!