I am a big fan of the CityMapper app, and never get lost any more. I ride my bicycle through the park an average of 50 miles a week, which means I frequently try new trails for variety and it has never steered me wrong, and finds the tiniest trails, almost deer paths, to get me from Point A to Point B. No matter where you are, it can lead you back to civilization, or at least to the closest street/metro station. As for vastness, you can start small by getting yourself to Peirce Mill or Veirs Mill (MoCo, but in the Rock Creek watershed), which has many simple trails nearby. And for a simple hike, again in MoCo, hie thee to Lake Needwood. The trail around it is, I think, 3 miles, and will give you a breath of fresh air and many gorgeous views of the lake.

(Side note: I have traveled to multiple US and European cities, and CityMapper has been available in each. Thank goodness, because I always used to get lost!)