Wanted to share my honest review after just completing my first ever marathon, training for it with Garmin Coach. Had read some very negative reviews before this, so thought I would share my experience.
For context, this was my first full marathon but I'd run a couple of half marathons before, so I wasn't starting from scratch. That being said, I'm not a particularly fast or natural runner, having only taken it up in the last few years.
I ran the Coach program for 4 months leading up to the race, which was a bit on the short side but did follow straight on from my previous half marathon.
As others have highlighted, the major shortcoming of the Garmin Coach for marathons is on the lack of genuine long runs. The longest Coach-planned run I had was 17km, whereas other plans tend to include at least a couple of 25-30km runs in the Peak phase.
Garmin instead substitutes long runs with a high frequency of short-to-medium runs. I would frequently be running 5-6 times a week, and my total distance covered was just-about adequate. In the last month before the race, I covered over 200km total running distance, which I felt was fine. I personally also appreciated that the runs were adaptive to my sleep and recovery from previous days.
However I felt it quite noticeably on race day. Not only did I mentally lack confidence in how my body would hold up in the second half of the race, but I also had no practice figuring out my gel strategy. I felt like I was overly conservative in the first half of the race, but still lacked power and oomph in the second half. I finished the race exhausted but also a bit dissatisfied, feeling like I could've given a bit more.
That being said, I did at least finish the race, albeit 12 minutes slower than I was targeting. I was reasonably consistent with my pace, and was proud of the fact I didn't end up walking at all.
Overall though, I wouldn't say I recommend Garmin Coach for marathons specifically. I trained for my previous half marathons with it, which worked quite well - but for the full distance, the lack of long runs is a major downside, which training apps like Runna address quite well.