I use the Garmin Self-Guided Training Plans (NOT the Garmin Coach plans) that can be found on GC Web (but not GC Mobile), and have had good success with the Intermediate and Advanced Heart Rate based marathon plans over the last 2-3 years.
But - they've got a lot of errors, some of which I've had confirmed directly by "Full Potential" (who created the plans that Garmin use), and some of which are obvious missing & incorrect steps in the Workout when you read the descriptions.
I've been manually adjusting these which doesn't take me long but is annoying when I miss one and then realise the instant I set off on the run.
I've reached out to Garmin support but got nonsense responses (two different workouts, one SHOULD be in Z1, one SHOULD be in Z2 - but they're both programmed as being in Z2 - suggestion was, "update your HR Zones on the watch" !) - and I've sent messages in to the "suggested improvements" place multiple times over a 2 year period and seen no change to the plans.
Does anyone know how/where I reach out to who maintains these plans within Garmin...the changes are incredibly easy to correct, its a minimal amount of effort - I don't know why they can't just do it !!?
THE PLANS ARE REALLY GOOD...I was a ~4:00 to 4:10 marathoner and really struggling to get quicker; since switching to the plans, I've ran 3:52 and 3:40, and I'm now looking at realistically getting close to 3:30 this spring 
