you definitely should practice this, and assume you won't get it right the first time or two so don't leave it to the last few weeks.
I'm fairly tall and heavy (188cms, 95kgs) and so what is right for me might not be for you.
I use gels only on runs beyond 2 hours, don't need them for shorter than that.
I take one at 40 mins and one every 30 mins after that. SIS Go, 60ml are my preference. Some may say they aren't needed so early and they may be right. If it is too early then no harm.
I also recommend experimenting with electrolytes during your training. Be sure to check what is available at your race so you can simulate in training. Personally I prefer just water and I carry my own electrolyte tablets. I choose Nuun brand. I take 1 at the 1 hours point and 1 at the 2 hour point. for runs of 3 hours or more
Surely none can be flatter than the Black Country half. It's a point to point race between Wolverhampton and Birmingham along the canal. There's not much terrain flatter than a canal.
It's scenic, the start is staggered based on your pace so there isn't much overtaking required, the start line is right next to the train station and the finish line is at Brindley Place, so no shortage of establishments in which to quench your post race thirst.
I'm sure plenty of people have done this and come to no harm, but I've got to think you would be exponentially increasing your chances of injury.
In large part it will depend on how used your body is to long distances. As this is your first I am assuming its not conditioned for this type of challenge. The marathon run at race speed really takes it out of you.
Speaking for my own experience, I finished my first marathon exhausted and sore but felt the worst of it had gone by day 3 and went for a 5k trot. Seems something wasn't quite right with the soft tissue of my foot and I'd tipped it over the edge. Took 3 weeks of rest to recuperate. I've never had an injury in my life before that.