How many days for these sites?

Will you be travelling by car or train/bus?
I see from you other post that you are looking for Harry Potter locations. For Glenfinnan viaduct you can ride on the Jacobite Steam train that was used in the movies from Fort William to Mallaig and back again, across the viaduct. From the viaduct you also have a view over Loch Shiel, that was also in the movies. I you are travelling by train, you can get to Fort William via the scenic West Highland Line from Glasgow. If you go by car you can drive trough the very scenic Glencoe valley.
Loch Ness I found to be a tourist trap. It is not particularly scenic and people only tend to go because of Nessie.
Alnwick Castle is very cool and not only for Potter fans. Nearby is the impressive Bamburgh Castle perched on a cliff overlooking the sea, but i think you need a car to get there. The inside is not as impressive as Alnwick.

I disagree with the previous comment about Loch Ness. I found Urquhart Castle very interesting and scenic. Yes, it's a ruin, but a well studied one with lots of interpretive signage, walking tours included in the entry fee, and costumed interpreters. Dress for the weather as it is outdoors. There are boat tours on the loch. A couple of miles from Urquhart Castle is the Loch Ness Centre & Exhibition, and indoor sight & sound experience that gives a fairly scientific and fact-based assessment of "Nessie" sightings.

Edinburgh Castle provides an interesting contrast to Urquhart since it's emphatically not a ruin but a well repaired, functioning complex of buildings. If you buy your tickets online in advance you'll save 1/2 hour or more waiting in the queue. To see the castle in all its nooks & crannies takes probably 2 hours. The rest of the day is easily spent following the Royal Mile downhill toward the Palace of Holyrood.

Apart from seeing Edinburgh Castle, you would do best with a rental car, and getting an early start each day (one destination per day, as you say).

I did not visit Urquhart Castle because the car park was completely full, so we decided not to bother with the crowds and drove on. Judging from what I did see and what I have read, the Castle is not unique and we had already seen a few nice castles, like Doune and Dunstaffnage, that were devoid of crowds.
In my opinion the only reason to go to Loch Ness is if you are into the monster stuff. Don’t go because you think it is the quintessential Scottish Loch experience, there are plenty of other Lochs that are way more scenic, nicer castles and way fewer tourists elsewhere.

You will need a day for each, as they are all in opposite directions!

There are far nicer, less touristy lochs to visit than Loch Ness and far better castles too, but each to their own.

A car would be beneficial for visiting Alnwick, then you could see some of the stunning Northumberland coast that you would miss if you went by train. Holy Island is stunning if the tides permit and there are some excellent castles too.

Thank you all for the advice! My husband really wants to go to Loch Ness. I could take it or leave it, but Urquhart Castle sounds interesting, and I do enjoy costumed interpreters. I hadn't thought about stopping or driving along the Northumberland coast when visiting Alnwick Castle. That sounds lovely. And yes, I am a Potter fan, so some I try to get to some of those sites each trip, but I try not to focus on them for my husband's sake. Perhaps we will spend more time in a car so that we can add on out-of-the-way locations.