I've been thinking of my own version of this:
Day 0: Train from NYC to New Brunswick, ride thirty something miles down the D&R and get a room in Lawrenceville just inside 295 after leaving the trail at the Bakers Basin Rd. Bridge. Did all but a mile of this last weekend as a round trip, it's a bit rougher and messier southwest of Princeton but not bad, the rest of the D&R was just like it is in summer, occasional puddles but otherwise fine.
Day 1: Ride through Trenton just after dawn figuring that's the best time. (Optional plan: don't do the day zero half ride but just take an early train to Trenton). Ride a bit over 68 miles up the NJ or PA side (TBD) and get a room in Bethlehem.
Day 2: Ride 60 miles north to Whitehaven PA and then 4 miles east on the shoulder of rt 940 and get a room just west of 476
Day 3 (wimpy) get up late, break things down, hike to the McDonalds parking lot and take the Amtrak connection bus to Philly and then train or bus back to NYC
Day 3 (determined) ride back down to Allentown, have dinner, break things down and take the 10:40 pm greyhound Allentown-Easton-Newark-NYC bus.
What I like about this is that if the second day were going badly, I could just hang out in Allentown or even Easton and take the bus back. And the last day riding south is optional, though very tempting as going home by way of Philly is quite indirect.
It seems a little silly to start with only a half day, but to do a full day from the D&R would mean going through Trenton at a busier time of day, and then there's not a lot of affordable lodging option in the region north of it. There's a possibility west of New Hope after 55 miles from New Brunswick but I don't relish the mile on rt 202 after the end of the Solebury Township Trail and it would make the next day up to say Leighton rather long.