I did this several years ago and it is fabulous!
I'd suggest at least 1 night in Padua before the tour starts. Frankly, if I were doing it again I'd do 2 nights so you have about 1.5 full days beforehand. I loved the Botanical Gardens (right near the hotel as I suspect you'll be staying at the Hotel Al Fagiano) and was not able to spend enough time here. I would also make a separate reservation for seeing the Scrovegni Chapel. You see it on the tour but it is SO amazing I wanted extra time there. You are restricted to about 30 minutes inside the chapel after a short time in a closed room - for dehumidifying I think.
Since you haven't been to Venice I'd do 4 nights there which gives you 3 full days plus your jet-lagged arrival day.
I spent one night extra in Orta and that was unnecessary and overkill. Really, you can see everything you didn't see on your arrival day in the AM, so a planned departure after lunch would be good. As transportation out of Orta is slightly difficult the guide will start ahead of time to get people organized. We were 3 traveling together so we wound up with another couple in a mini van to Milan which worked well. Our guide arranged taxis for those that wanted them to Milan and Malpensa airport and helped others ahead of time to purchase train tickets to other destinations as there is not a manned ticket station in Orta.
I spent 1 night in Milan and should have spent 2 so I could have gotten tickets to see The Last Supper.
When we started in Padova I went in to the Basilica of Saint Anthony and realized they had his relics there I sort of made that my theme for the trip. I tried to see all the various relics in the cities we visited. That may gross you out!
Do spend some fitness time on stairs or hills, depending on what's available where you live. There is a lot of climbing on this tour!
editing to add: About departure....this was the time I decided no more early flights for me either. Getting from Milan back to the Western US required a few stops and I left Milan Linate at 0600. I decided in the future I'd fly to AMS one afternoon and do at least one overnight there before a flight back to my SLC hub. Of course you may have better choices from CT than from the back of beyond, lol!!