You’ll be fine and have a great time! In the right weather the Pony Track is not especially hard, just steep and long.
Buy a paper OS Map. Cut out the relevant section beforehand so you’re not trying to unfold a massive map in the wind. Also download an app with OS Maps (you’ll probably have to pay). You can do that through OS themselves or another app like Outdoor Active. Use the app as your primary navigation with the paper map as back up. Make sure your phone is fully charged. Don’t be tempted to use Google Maps or similar.
Take a compass and watch some YouTube videos on how to use it.
There’s a website called Mountain Weather Information Service. Check this the morning you go. It tells you what the weather will be like at the top of the hill. Other weather forecasts tell you what the weather will be like at the bottom. This is often very different.
If the MWIS forecast is bad, don’t go. This is actually quite important. Ben Nevis by the Pony Track on a sunny day with low wind is not that hard for two fit people with good legs. Ben Nevis in thick cloud could kill you (it could also kill you in mild sunshine, but it’s much less likely).
Buy a rucksack of at least 25 litres. Pack 1l of water (each, minimum, personally I would take 1.5l), a good waterproof jacket, a warm insulating layer, a map, a compass, a headtorch each, lots of food (preferably about 1000 calories each). Lots of carbs and sugar and fat. Today is not a day to be healthy. Eat gradually over the day rather than stopping for one massive lunch. Eat while moving.
Leave early! The days are shorter now. Work out how long you expect to take, then add 50%. Leave early enough that with that contingency you’ll be down before sunset. The head torches you’ve packed are a precaution in case you’re not.
Tell someone where you’re going, where you’re parking, your number plate and a description of your car, the route you’re taking, when you’re leaving and when you expect to get back down. Tell them to call mountain rescue (phone 999, ask for police then ask for MR) if you’re not down by your contingency time (which, remember, is expected time plus 50%.) Text them when you’re back at your car to tell them you’re safe. If you’re delayed but safe, text them once you have signal to tell them so. Don’t expect to have much signal.
If you start but it’s all going wrong, either because of weather or injuries or something else, don’t be afraid to turn back. The mountain will still be there next year.
Remember that most accidents happen on the way down, not the way up. Peak time for mountain rescue call-outs is about 4pm. Leave lots of time to get down without hurrying.
Book a table at a nice pub afterwards. Nobody wants to be standing in a massive queue when you’re ravenously hungry and your feet feel like they’re about to fall off.
I know that sounds like a lot. It’s not meant to be scary. You’ll have a great time. On a good day the views are stunning. Be safe and enjoy it.