Description ultimately revolves around the use of imagination. In a paragraph like this, I'd suggest you think first and foremost in terms of setting and imagery. As the assignment establishes, you're walking in the woods on a hot summer day. Even so, what are the conditions like? Is it humid or is it dry? Is there any cloud coverage? You can also write about the forest itself. What kind of trees and plant life do you see? For example, an old deciduous forest is going to look very different from a grove of evergreens, or of sequoias. What kind of wildlife might you observe? What does the trail in front of you look like? Does a trail even exist?

There are more emotional and experiential elements as well; for example, how do you and your friends emotionally respond to the forest around you? Moreover, consider that your experience would be very different if you've already been hiking through these woods for several hours than if you had only just begun your walk. Perhaps people are getting tired or frustrated, especially if they've already been walking for hours and are ready to turn back. In any case, descriptive writing, as said before, relies on a very strong imaginative element, built around the imagery you are able to create. Beyond this, think about the larger narrative context in the background to give this writing structure. How long have you been walking? Are there any conflicts in your friend group? These two components should probably be the most important factors in shaping your response.