Editor's note: We asked noted entrepreneurs to reflect on what they wish they'd known starting out and to put it in a letter to their younger selves. Tom Gimbel is a founder and CEO whose LaSalle Network ranked on the Inc. 5000 list of fastest-growing private companies in the U.S. 10 years in a row.

Dear Tom,

When it comes to advice, there is truly only one thing you should know--and it's an obvious one: Get in shape.

You have the work ethic. You have passion. You like people. But you will never get a second shot at being young and laying the foundation to be in shape. It's not about getting those 6-pack abs and being able to bench press 250 pounds. The main objective is just getting shape: Running five miles easily, not having lower back pain, being flexible, and not having tight muscles and ligaments. Having a strong core, as they say.

Right now, you're in your early 20's. You don't sleep. You are up late into the night. You work late and are growing your business. Believe it or not, it does affect your work and family life.

Mind and body are connected. At 15, 22, 30 years, you might not think so because you feel great. (Don't be an idiot!) Look at it from a business perspective, and think of the lost hours you'll be spending at the doctor's office, at physical therapy, and the emotional drain of feeing like crap. Laying on the floor of the office with an ice pack because of a flair up is not an ideal way to spend your day.

Do yoga. Pilates. Get flexible. Strengthen your core. Stretch every day. Run. Lift weights. Sweat.

"But it takes too much time..."

Well, that's B.S. because it's not going to take more time than going to the doctor, or leaving work early because you're in pain. There is a direct correlation between the physical and the emotional.

How much more productive could you be? A lot. You're already up at 4 A.M., anyway, so start hitting the gym. If you can't force yourself, invest in a trainer. The discipline you have outside of work will benefit you in the office. You will set an example of how to live to your kids beyond your work ethic. You will do 5K and 10K runs with your friends. You will feel better.

One day, you will find out that your peers and competitors were actually meeting clients at the gym. Do fitness activities with their staff and employees. The list goes on. The point is, you limited yourself by not giving yourself time to be healthy.