If you're not certain of how far you can hike before it becomes too much of a strain, hike in for a predetermined amount of time, and then hike back following the same route. So hike in for 15 minutes, and hike back for 15 minutes. This will save you if you are worried about the intensity of the trail, how much time you have, how cold/hot it is, or any other factors that may make you hesitate on going on a longer hike.
I often do this when I know I don't have a lot of time. If I only have 30 minutes, I'll walk in 15, and out 15. If you see that a trail is many more miles than you want to hike, you can easily do the same thing. Just look at what time you started so you know what time to turn around. This way you can still see amazing things and go on a great hike without worrying about overdoing it, or keeping to a time limit. So go out, enjoy some nature, and conquer some trails!