I'm early 30s, 20 BMI, gym a couple times a week, walk everywhere, but have never been much of a runner. I've been trying to get my fitness/capacity up on a treadmill, but I know that running outside is quite a different beast!
Large wall-o-text incoming.
Most runners will have some strong preference for either outside or inside running, outside can be harder since you don't have the band going by itself underneath you, but if you do some incline fast pace running on a treadmill you'll get to the point where it's just as hard. I prefer outside because it's less constant and forces some adaptations due to various surfaces and weathers.
If you want to just gauge your fitness levels, you'll first need a HR monitor of sorts and to determine your HR max, which can be difficult, but for you as a beginner you should be able to spike it to max in an all out effort. Most sportswatches will give you a VO2max figure; various brands have their own tests too, Polar for example has a Fitness Test that you can do at rest (might not map well on running VO2max), Running Test and more. HRV should give you an estimation of cardiovascular health/fitness too, something like a cheststrap works best for that.
As for your HM, early 30s with that BMI you should have a relatively easy time. How much you weigh is very important for running, as the heavier you are the more work your heart and muscles must perform to push you ahead, and 20 BMI is pretty light. If you haven't run yet it'll still be (very) hard for a week or two, and will slowly get more and more pleasant as your body adapts - you'll notice that in your lowered HR during runs at the same pace and plenty of other adaptations.
Basically all brands that have running watches have plans you can follow, from Garmin to Coros to Polar and more. Many of these plans are well done but might be difficult to follow for beginners, because they'll ask that you run long runs and easy runs inside your lower HR zones, and for beginners that's just not gonna be possible, you'll have to either walk or just accept that for months (depends on your levels of couch potato) your runs, even slow ones, will be in the higher HR zones and even a jog will get your legs heavy with lactic.
You can just follow one of those plans, there should be apps you can download that do it even without a wearable, but it's better if you can see your HR to more accurately estimate effort.
Another option is to just go by ear, and instead of delaying "until the proper time", just start running. Get some proper shoes. And go out and go. Increase mileage slowly by 10%-ish per week. Control your pace, don't go all out, your hard runs should be rare and rather short, 20% of your training. Most runs you should be able to maintain for an hour and speak through it, even if your breathing is deep and labored. Conserve energy, your goal is to last long enough, increase resistance to fatigue, adapt body at dealing better with lactic acid.
Running form and efficiency are very important too. You'll slowly learn a lot of tricks, you'll learn to land on the ball of your feet, not on the heel, you'll learn to not overstride, to increase cadence, to stay tall, proper hips and arms movement. All contributes to your form, and bad form leads to injuries and less efficiency. With fatigue, form decreases, so it gets hard to maintain over longer runs.
Don't go fast. Just go chill even if it looks stupid and other runners whizz past you. Give yourself a few good months to adapt to 20km runs. 2-3 months in you should be able to run HMs alone so you can understand what it takes. You have to understand the pace you can sustain, or you'll hit a wall if you go too fast in the race, and either give up or walk to the end like a zombie.
Oh, and most running problems can be solved by more running, that's most important I think. 5 days/week is OK. As you get more experienced you'll be able to run 6 days, and if you get good, you'll be able to run each day, and recover through lighter, short runs, instead of not running. Keep in mind that detraining is quite fast, especially in beginners. So if you run for 1 month and make good progress, and take a week off, you will have a few worse runs for sure.
Eat and sleep well. Running is very taxing for the body. You have to give your muscles and cardio system time to recover. You'll be able to measure fatigue through HRV.
There are a million things to say about running, but for your HM, simply get shoes and start going out to run.
Be careful with the shoes you choose, if you can visit running store, do so. They have to be a bit bigger in size, if you get them with a very tight fit your fingers and nails will suffer. Badly - loss of fingernails and eternal black nails and blisters are common. Other injuries like shin splints are also common with beginners.
Best of luck and let us know what you chose to do
