Half marathon under 2hrs is quite a goal 👍That was also my very first goal for HM (I trained to finish at 1:45 to 1:50).
Fitbit unfortunately doesn't come with training features so you will need to look at thirdparty software (and maybe hardware). You will find good training plans in Strava (it may require to subscribe, I don't remember). TrainingPeaks is a great tool (includes training plans but again - you need premium subscription). TP is probably best platform for such training.
From my personal experience, the best results I have from training with Stryd but this is software and hardware (foot pod device you attach to shoe when you run which measures your running dynamics). Instead training by pace you train by running power which changes a lot. Stryd has plenty of workouts and training plans, too. It works together with TrainingPeaks (TP has running power support). I use TP to shape my pre-race effort so I know I'm not over- or undertrained before race. In general, I use mix of software and hardware to support my running.
For my first HM, I used Garmin training plan (it integrates workouts with watch so it's easier to train) and it was good. I completed that race under 1:40 so better than planned. For next one, I did one of Strava plans but this one is tricky, because you need to make sure your pace zones are correct (ideally, you need to do at least all-out time trial for 5k, as if you race, then enter time in Strava and it will figure out your pace zones). Strava plans without setting pace zones won't really work as you won't be able to tell what effort you should be running with (it's all descriptive so zones help a lot).
Mind that most of the software opens training plan features after subscribing as premium user but well... sport isn't cheap 😁
It's good if you look around and find your own set of tools that suit you (rarely one tool fits all training needs, at least not in my case 🙂).
