I was at jiujitsu yesterday and there was a young guy trying out his first class. Early 20s, no body fat, lots of muscles, been doing karate and other sports his whole life. Towards the end he was so exhausted he could not finish the full class. At the end he said, "I have never been this terrible at anything athletic in my life."
Right with ya bro. When I first started I thought I would do pretty well. I was in great shape, strong, quick, good cardio. Or so I thought... That first class completely wrecked me! And continued to wreck me for weeks on end until I actually learned something.
You have to learn how to pace your yourself, how to breath, strategy, positions, submissions, etc... You are not efficient, it is confusing, everyone else makes it look so easy, and it can easily demoralize you.
Because I was getting crushed by people I have 30, 40, and even 50 pounds on. Just looking at some of these guys it looked like a complete mismatch, then next thing I know I've got a spider monkey on my back choking me and I don't know how we got to this point.
Back to yesterday, as I was walking out I saw some pads that said, "Earn your ability" on them. Right on pads!
The guys that were beating me did not just walk in and start dominating. They put years and years of consistent blood, sweat, and tears into getting there.
I see people get discouraged by this same thing in my classes some time. They will see women twice their age out there crushing their workouts and running circles around them. They get discouraged by I have to remind them that these women have been coming for 5 years! They did not walk into class the fitness beasts they are today. They earned it.
Don't compare your page 1 with someone who is on page 212.
So how do you get there? Here are two simple, but crucial tips to help you earn your ability.
1. Learn to push and pace
There is a delicate balance with these two that is different for everyone. You can't go in there and go all out so hard that you can't finish the workout. And on the other hand you can't go half paced all the time.
Use weights that challenge you, but don't destroy you. Do your cardio at a pace that you can maintain for the allotted time and distance. You will learn this through the next step.
2. Be consistent 
You did not get out of shape overnight, you will not get back into shape overnight. This is my number one tip (despite it being listed as number 2), that I include in almost any list about getting better. Because it is so crucial!
If you kill it for 3 weeks then you take the next 6 off, you will not get anywhere but really sore each time you start back. It is better to consistently do 2-3 workouts/week for a year, than 6 workouts/week for a month. You will learn more about your body through consistency than just about anything else.
If you have already been exercising, but you aren't where you want to be look at these 3 things. Which one do you need to work on? Do you need to push a little harder? Back off a little so you don't wear yourself out before you can finish your workout. Or do you need to show up more on a consistent basis?
PS- I've been doing a series of short videos on the Huntsville Adventure Boot Camp Facebook page. If you would like a few extra tips or if you just enjoy ginger eye candy, you can click on the fancy red font there and check them out.