
If you just go into the gym without a plan you are MUCH more likely to not push yourself past your "easy" point. I always sit down and write out a whole workout before I even think about going out and doing something. Then when I'm at the gym I have to push to do everything I wrote down, and its also much more methodical workout, keeps you from jumping to far off your goal.
For instance today I was thinking about heading to the gym, but it was so nice outside I decided to figure out a workout I could do at the park for my lower body. I knew I had to stray from some of my regular workouts because I didn't have weights to use.

Here's what I came up with....
1 min regular squats
1 min step ups right leg
1 min step ups left leg
1 min jump rope
1 min sumo squat
1 min regular squat
1 min jump rope
1 min push ups
3 min walking lunges
1 min jump rope
1 min butt raise
30 sec right leg raise
30 sec left leg raise
1 min jump rope
Then I did a full ab workout...about 7 mins total
But having it written down really helped me make sure I pushed through any tuff points. I ran to and from the park for a warm up and cool down and just carried a jump rope with me. You can always do jumping jacks instead.