With no running background the most important thing to do is to start slowly to build mileage. Increasing mileage too rapidly is the biggest cause of running injuries. To condition the soft tissues of your legs (tendons and facia) takes many hundreds of miles. I use a simple rule that you should not even thing about starting to train for running until you have accumulated 100 running miles on your legs. Say you are putting in 10 miles/week now then you can expect toe be able to start real training in bout 10 weeks. Now, if you increase your weekly mileage do not do so faster than 1 mile for every day you run in a week. Say you run 4 days a week then you can add 4 miles to the weekly total. These may seem like overly conservative numbers to anyone who hasn’t run much but the running landscape if littered with injured runners who tried to ramp up too fast. You are young and at least 12+ years from your mountain running prime. Use this time wisely to establish good running form and lots of base miles and you could have a very fun and long career.
As for how fit can you become. No one can tell you this. This is a sport where grit, motivation and the ability to suffer count almost as much any physical gift. Just don’t go hog wild in this first year or it could be your last.
I hope this helps.