Despite their drawbacks, the Garmins take sufficient track-point readings to avoid making "as the crow flies" short-cuts in their measurement. The errors they make are cumulative over the whole race, unless the GPS signal is lost (when they will take the shortest route between the point of signal loss and signal re-gain). This is very unlikely however, especially with the newer 205/305/405 models. I have been using the newer models for about 2 years and only experienced a signal loss once and that was during heavy snow-fall.
How about that - from Garmin attack to Garmin defence?!
I can't answer for the positioning of the mile-markers, but my message for Garmin users (NB I am one myself) is to stop blaming them for failing to meet your race goals.
IMO - the course was accurate. The organisers of this race will have ensured that - they have been putting it on for years and they knwo what they're doing. Please note that GPS sports watches are not surveying devices, which is why the people who measure and certificate the accuracy of courses do not use them for that purpose.
For what it's worth (and bearing in mind my commenst above) my Garmin measured the course as 13.13 miles. That's an error (in the Garmin not the course) of 0.02 miles or about 32 metres.
My tip would be that if a PB is your aim, you should (for safety) assume that the Garmin will over-measure the distance (sometimes it will under-measure too). I look at the average-pace read-out on the Garmin. If a PB equated to 6:30 mile pace for example, I would for safety aim for about 6:28 pace on the Garmin. In my experience, the Garmin might get the average pace wrong by a couple of seconds either way due to slight over/under measurement of teh course. They are a great pace-guide tool, but don't relay on them too much! In longer races a couple of seconds per mile error in the pace can add up too, so be especially wary in a Marathon!
This is my first half-marathon, so exited. but a bit nervous. Only my 4th ever race (done 3x 10k previously). Did 10 miles in 1hr 13m in training last week, so thought, what the hell, it's only another 3 miles and Stevenage is local!
Weather forecast for Sunday is Sunny, max 13 degC and low wind. Sounds better than last 2 years!