Thank you No1philatelist, but Jean from NJPHS really deserves much or most of the credit. She research this over a couple of days (got quite excited in fact!) and left me all the bread crumbs.
For those interested, here's my transcript of the letter.
New York 18th July 1811
My dear sisters, I suppose you begin to feel anxious to hear from us and to know how we got home. We had a very pleasant time after we got over the mountains. When we passed over Long-Hill we were very much delighted with the beautiful and extensive prospects. We arrived at 'Square' Littles about 12 o'clock, took a little gin sling. (?) and started for Newark where we landed safe and sound in wind, limb and eye sight, a half past 2 o'clock p.m. We found mother much better than we expected, however she was not well. Sunday afternoon we rode down to Elizabeth Town, spent the afternoon there, very pleasantly. I returned about sunset; we had a delightul time indeed. When we got home we thought it time to begin to prepare for N. York, a ???? we fitted ourselves out for a trip, which, "I guess was nicely done to it" and took our departure on Monday morning. We arrived safe at half past ten o'clock in this great City of Gotham, where we were heartily wellcomed (sic), and our return greeted with hard shaking of the hands and smacking kisses - well ; now you see we are safely home – but, to return to Lornerton (?) Lord bless you, girls how do ye do? Is Emma's stomach any better? Does our friend Betsey Parker remain in good health? I should like to have a frolick (sic) with you all this evening - but alas!
Richmond says he will write to Emma and prescribe some remedy for her sickly body. I hope the Somerton/Lornerton air will have as good an effect upon her consumptive habits as it did upon us. - Oh! my/with a sigh/we wish for another jaunt. We were so well pleased with the last. But I don't know when we shall come again. Perhaps we shall be able to come next winter, when you all will be at leisure. – When we reflect upon the pleasant time we enjoyed at your house, going and returning from Schooley's Mountain and we really feel ourselves much indebted to you and heartily wish for a return of so happy a time – We now see the difference between the city and country and we are firmly persuaded, if you could realise half the blessings which Providence has profusely lavished upon you, and enjoy the comforts within your reach – you must be happy – if you are contented. A contented mind is a continual feast – but as the fellow said, "there's the rub"_ However I hope you can philosophise (sic) enough (with the numberless enjoyments which continually surround you) to be perfectly contented and (as far as this world affords) perfectly happy –––
(turn over)
Well what more shall I say – tell Em, " I feel queer all the way down" and I should like to wet her head with a little cool water –
Give my sincere love to your "husband" with many thanks for his kind attention. It is uncertain when we shall repay him, but if we are unable to requite his kindness, we will come again and increase the debt and some time or other, pay you all a lump, for we have no notion of doing business on a small scale. The Schooley's Mountain has almost cured Sarah, and I have no doubt but that the jaunt and water together will have an
admirable and good and great effect upon Mrs. B and if "I had only time enough I'd tell you all about it" – strike up the music, Yankee Doodle.
Give our sincere respects to the old Gentleman, with thanks for his politeness and kindness. I hope his horse will arrive safe home, we are much indebted to him for the use of it. I gave directions at home to have him well taken care of. – remember us to Wm. (William) – tell him we shall not forget him as long as a twig grows on the Lornerton Farm.
I believe I have written almost enough and I will leave the rest for my worthy friend, Madam B.
Emma must not forget to bring that book home with her and send it to me. I never thought of it when we started – Upon the receipt of this, we shall certainly expect a long answer from each of you.
I hope you will have a prosperous time in gathering your harvests and I trust Heaven has in store for you and your beloved husband, many returning "Harvests" of prosperity and rational pleasure, and many feasts to come of joy and mutual love.
Accept the sincere well wishes of your affectionate
Horatio
"Farewell"
n/o
Mrs H. Duyckinck and
Miss Emma Holden
Lornerton
Emma must not forget the book _
notes on back
Mr Duyckinck's tickets are all blanks except No. 2.950 - a prize of $10
Direct you letters No. 26 Nassau St. care of B. E. Bliss
it would be better to send them by Mr Duyckinck whenever he goes to Newark and father will send them to us thereby save postage
Image showing notes.

Me again! Jean also noted that the reference to New York as 'this great city of Gotham' as coming from a Washington Irving book, the Salmagundi Papers in 1807.
She also figures 26 Nassau St. is probably in New York City, as there is not Nassau St that we could find in Newark. This address in NYC is now Chase One Manhattan Plaza in the Financial District. Couldn't find any reference to anyone named Bliss living there 200+ years ago.
I found a period map (1812) of New Jersey which shows some of the landmarks around Lamington. Also regarding the discrepancy in the letter writers name (Horatio/Horace) I found several references online to the 'fact' that at one time these names seemed interchangeable. Reference Horatio/Horace Walpole, 4th Earl of Orford.