Loch Ness has a good reputation but is quite hard to get to, especially if you need to be back the next day to take the kids to school etc, it's a good 4 hour drive from Glasgow or, if I recall correctly, you need to be on your toes and back to the airport pretty quickly, the last Birmingham flight for example is 17:30 although there is a London flight after 8pm. Certainly a few years ago you had to be there the day before to regsister too. The flights are that expensive, book now and you'll be under a hundred quid.
For an autumn marathon I'd say Chester is a good race, flat(ish) with a couple of short hills, you'll struggle to find a better organised race, good parking, close enough to the city centre for your family to see you off, have a brew and a wander around and then get back to the finish, pretty reasonably priced for a closed road event too at about 40 quid.