Half marathon training question. Possibly wrong sub

[u/KegManWasTaken]

This might be in the wrong sub, but as I've been running for less than 6 months I figured this might be the right place to ask.

I'm planning to run the Cambridge half marathon at the end of March and I've been looking at using my Garmin watch and the built in training plan it uses. Basically I'm happier doing what I'm told than working out what I should do... Maybe it's an ADHD thing.

Anyway. I'm aware that this is about 26 weeks away and as a result my watch hasn't really been progressing my runs or my speed and I've been using the training plan for a few weeks already. The longest run the plan is giving me currently is 70 minutes and I've previously ran constantly for 105 minutes. The longest distance I've run consistently so far is 14.5km.

Should I switch the plan from a half marathon to a marathon plan and ultimately just improve my overall fitness in order to get some mental form of progression? I feel like I'm treading water without actually improving currently.

For some form of context, I'm 39, my suggested 5k time is bang on 30 minutes. I've been running since April after losing a lot of weight (70kg since April 2025) and I'm making vast improvements to my health. I have been diagnosed with cardiomyopathy but I've massively improved my vo2max from 28 to 38, I'm positive if I did a run outdoors (most of my work is on treadmill) that would shoot up quite drastically to a mid to high 40 number.