First of all, 30 is not mid-life, so it's not a mid-life crisis! And 30 isn't aging...70 is aging. 30 is just a number, and it's a good number. The best years of my life so far have started with a 3.
For me, it's all about keeping perspective. My best friend died in a car accident the weekend we graduated college. Normally I would have been in the car with her. So every day after that, even the ones that were scary and sucked (I was a single mom for 5 years so there were a lot of those) has been a gift.
If you are unhappy with where you are in life right now, picture where you want to be in 5 years. What does the ideal of 35 look like to you? How is it different from where you are right now? What steps do you need to take to get from here to there? Then start to plan and do. It doesn't always work out neatly, but having a vision of what you want and a plan to get it are a start.
When my oldest son was born (I was 22), I knew that I had no interest in dating anyone for a while and wanted to just focus on being a mom and getting my feet under me, balancing work and single motherhood. I also knew that by the time he was 5 and starting school, I wanted to live in a town with good schools where I would want him to spend the rest of his childhood, that I wanted more children so therefore would want to get married eventually, that I wanted to own a home, and that I wanted to have enough flexibility in my job so that I could work at home at least some days and not have my kids in before- and after-school care every day. I actually have all of that, one step at a time. The first step was that after two years of working at a place that was awesome and supportive of me when my son was an infant but a long commute and required that I work on site, I changed jobs to something closer to home and that has become my career and after a few years there, started working from home 3 days a week. I went from living with my parents for 3 years to renting for a couple to buying my own home. I met my husband when my son was 3, got married when he was 5 and have two more children with him.
Just try to keep perspective about this birthday - you are young and have your whole life in front of you. People start and re-start their lives at any age. I have a friend whose husband was overweight and on medication for high blood pressure and high cholesterol in his early 30's. 15 years later he's a fierce cyclist - he bikes 50 miles a day round trip to work, does century rides (100 mikes) on a regular basis as well as mountain bike races and he's almost 50. I know a ton of people who started to take their health seriously in their 30's and are in the best shape of their lives, running marathons, doing triathlons, or even keeping it simpler with yoga and jogging. Your 30's is also a good time to go back to school, change careers, have children, etc. It's a decade of maturity (in a good way) and change. Of settling in to who you really want to be and worrying less about what others think of you. Really, enjoy it!