The onset of a female's first heat is usually about 6-9 months of age for large breed dogs, but it can vary. I doubt you need worry about the incident at the park, puppies try to hump each other all the time at this age. However, it is good to be watchful until you get her spayed.
Usually, the female will go through a very heavy shedding period a few weeks before she comes into her first heat. That is the first sign. Next you may notice her licking herself down there, there may be swelling, and then blood. At this point don't take her ANYWHERE and keep her under lock and key at all times so that no males can get to her. Heat usually lasts about three weeks, but it can vary, so it's safest to keep her on lockdown for about four weeks.
If you don't feel you can adequately protect her from getting pregnant, get her spayed ASAP.