Astoria to Boston is 3200 miles via US 30, I-84, US 395, and I-90.
Taking cycling roads and hitting places like Yellowstone and Rushmore will add 15% to 20%.
The Northern Tier is 4300 miles - granted it goes to Bar Harbor and has some extra loops.
But I think you should plan on a minimum of 3800 miles for a trip from Astoria to Boston.
That said - 60 days ain't a whole lot of time.
A bridge engineered for 61 tons isn't rated at 60 tons.
You need to allow some leeway - for glitches at the start of the trip -
For weather, for Montezuma's revenge, for sore knees or tushes.
You can always take a few lazy riding days at the end of the trip if you are running ahead -
But nothing ruins a bike tour more than constantly trying to play "catch up".
I'm also not sure how your travel days fit into your itinerary -
But let's say you have 60 riding days after you arrive in Astoria.
That's 8 and a 1/2 weeks.
Some people don't like taking whole days off, but do light days.
Either way - give yourself one day off per week for all exigencies.
That makes for 52 riding days to cover 3800 miles.
By "old math" that comes to 73 miles per day - quite doable - but tight.
I agree with others who say that you should keep the first few days fairly light.
It doesn't make sense to blow out your knees right at the start.
So you will be looking at 75-mile days for most of the trip.
Best - J