Editor's note: Annette Miller lost more than 100 pounds before joining CNN's Fit Nation Triathlon Challenge program. Follow the "6-pack" on Twitter and Facebook as they train to race the Nautica Malibu Triathlon with Dr. Sanjay Gupta on September 8.
(CNN) -- It's New Year's Day 2013. I can feel the wind on my face. My heart rate is steady, but I'm so full of joy and pride it may beat out of my chest.
I can hear my friends yelling from the sidelines: "You did it! We're so proud of you!" I try to choke back tears. My knees buckle as 14 people surround me.
I have just finished my first 5K race.
But wait -- something is missing. My two best friends are nowhere to be seen.
"Your relationships will change." I heard that piece of advice more than any other during the Fit Nation kickoff weekend. I think about those four words often.
I was always someone a friend could call at a moment's notice to go out. When I finally decided to get healthy, I couldn't continue to do that. I made a conscious decision to change where I hung out, who I hung out with and what we did when we were hanging out.
The majority of my friends are health-conscious, so I didn't think any of my relationships would change much. I was wrong.
Going out to eat is a rare occasion these days, and I can't remember the last time I had a drink. All of my free time is spent at the gym or doing something active. To maintain my active lifestyle I make sure I get a minimum of seven hours of sleep every night. Most of my friends are going out when I'm going to bed.
I knew my best friends and I hadn't seen each other in a while, but in all the years we've known each other, we had never missed the "big stuff." That morning, after my first race, I checked my phone. There were messages from both of them telling me how proud they were, but that New Year's Eve had gotten the best of them.
Later, I explained how hurt I was that they weren't there. I found out they were hurt, too. They felt like they didn't "fit into" my life anymore. We had always spent time together seeing the latest movie, checking out a new restaurant or going to a concert. Vanderbilt basketball games were our favorite, but most of those games were at 8 p.m. Thursdays and I was in bed.
The Annette they knew and loved was changing.
It was hard to hear. But we talked it out and -- because we're women -- we had a really good cry.
Any time there is a new challenge in your life, be prepared for your relationships to change. Your friendships will change; your family life will change. And don't be surprised when the most significant change is your relationship with yourself.
I thought the absence of certain people in my life was temporary. It's not. The changes I've made and continue to make are permanent lifestyle investments in me and my ultimate goal of living the rest of my life as a healthy, happy, active adult.
I've made new friends since the beginning of my journey. I'm a social person so I'm always going to have a group around. Some of that group has changed because I needed it to, but those who matter have weathered the changes with me and will always have my back no matter where life takes us.
My friends and I still don't see each other as often as we did, and probably never will. But we check in on each other and have made a promise to hang out at least once a month.
When I cross the finish line of my first half-marathon on April 27, I know both of them will be there with smiles on their faces, arms open wide and pride in their hearts for whom I'm becoming.
Follow Miller on Twitter @TriHardAnnette.