I'm in for Loch Ness 2013. I did it in 2012 as my second marathon (first being Edinburgh 2012).
I trained very badly for Edinburgh and marginally better for Loch Ness...
The hill at 18/19 miles at Loch Ness hurts if you've burned yourself out early (as tricialitt mentions) on the downhills and the hill at about 22/23 miles is almost as bad. I stupidly went faster than I planned in the first 10 miles or so which are generally downhill and then was back on average pace over the next (flat) 6 or 7 miles. I toiled badly from about mile 18 to mile 23 (ish). Even with that, I really enjoyed the marathon and signed up for 2013 within the next few days! Finishing time was 4:24.
I've been training "properly" since the start of this year for 10k distance and now half marathon distance. I'm looking to knock at least an hour off my marathon PB this year...
As has been said - if you don't have accommodation already, you'll need to get on it soon as the accommodation fills up very quickly. If you've got anyone going with you in a car that take any luggage etc. then an option away from Inverness is to stay at Drumnadrochit or Fort Augustus where there are bus pick up points. We stayed in Drumnadrochit last year and that worked well. Inverness City Centre this year though to save my wife (now with added baby) having to find parking etc. on the day. We'll be staying walking distance from the finish line!
Another tip. It was bloody freezing at the start line with a decent amount of time to wait before actually getting going. If you can take something warmish to wear to keep you warm that you're not bothered about chucking away at the start or soon after, then it would be advisable. The bag drop is good too - I went dressed in my hoody and jogging bottoms and kept them on for as long as possible before throwing them into my rucksack and onto the back of the trucks. The pick up wasn't far from the finish line and I had my joggers and hoody back on in no time after finishing. 