I’m thinking about doing a 150 mile (2k ft climbing) solo road ride this Saturday just because I have the time. How hard will it be assuming I fuel appropriately and stay in zone 2 and 3?
I’ve ridden 100 miles (4k and 5k ft climbing) twice in the last 12 months at a zone 2/3 pace. I was tired after both rides, but felt ok. I’ve been averaging 70-80 miles a week for the last two years and have been following a TrainerRoad plan for over a year.
If you’ve done a couple 100 mile rides you’ll be fine. You’ll be tired, but fine.
I would err closer to a Z2 pace than a Z3, given your recent mileage, but you’ll absolutely be able to do 150 miles if you keep on top of your fuel and maintain pace discipline. This will be especially important early when you feel fresh and want to “bank some time”. Don’t do that. 
Its a pretty flat ride by the sounds of it and as you’ve already done a few hillier ton’s in Z2/3 you’ll be fine.
Another you’ll be fine. But also: consider, if possible, a route that puts you on a downhill and/or tailwind on the way home. That always helps.
It’d be a big ride but certainly doable. Just focus on three things:
Turn off the power numbers screen and just go ride.
I would blow up if I did this. I’d go out way too hard at the beginning.
Then you DESPERATELY need to learn how to not ride with power.
General comfort will possibly be more of an issue than fitness. Aches and chafes become more of a thing I find. Shoulders, neck, hands, backside all might start to niggle.
But like all the other posts, roll out like you are going out for a big easy ride and you’ll be fine. If you can’t pace it off that kind of feel, then make a deal with yourself to keep your heart rate to zone 2. I wouldn’t even look at power. Just plain and simple don’t push on the hills, wait for the fatigue to come to you rather than you ramping up the effort from the base of a climb.
Ive done LoToJa 3 times (200 miles @ 8500 ft climbing with 75% in first 100 miles). We train for it by doing trainerroad rides during the week and 100 mile rides on saturdays. If you can do 100 miles, you can do 150. Just be careful to pay attention to heat and wind. Then make sure you have a good nutrition plan with easy stuff to eat and known stops. After about 75 miles, i cant really chew anything like a cliff bar while riding.
Why?
If you’ve done 100 then you’ll easily do the 150 - just keep eating, spinning, being mindful of your surroundings, and not checking the clock and you’ll be grand.
Hope you have a good ride and may the tailwinds be with you the whole way 
How hard it will feel will to a certain extent be subjective. Just go easier than you think earlier on and see where you are at, at 110 miles or so. Still feeling good, RPE hasn’t gone up for same effort, then you are in a good place for the remaining 40 miles. Try and finish feeling like you could do at least another hour or two.
Hard.
If you race up the early hills and or struggle to finish, then Very Hard.
If you chat throughout and feel a bit bored, then Moderate.