I work 80 hours a week, and I'm happy with my social life and fitness level. To be and to stay fit you need to eat right, exercise and... have good genes. Nothing you can do about your genes, but here're little hacks that helped me to not only stay in shape, but also lose 20 lb in 6 months and maintain it. There's nothing new, it's very easy, but you need to be disciplined about this.
1) Eat 4-5 small meals a day with plenty of veggies and protein. Exclude any processed food and ANYTHING with corn syrup. Read the ingredients. If there are words that you don't know - don't eat it. I never starve, never diet. But I also don't eat more than I need. If you drink alcohol, try to stay away from beer and cocktails. When I drink socially, I usually either have a glass of wine, or go straight to hard liquor (cognac, tequila, vodka).
2) Work-out 3-4 times a week (every other day) for only 20-30 minutes. To be effective you can work out with you weight: push-ups, pull-ups, sit-ups crunches, etc. The key is to go with no breaks, and do 3 sets to maximum repetitions, while alternating different exercises. When I'm short on time, I do 10 pull-ups, 20 jumping push-ups, 30 jumping lunges, 40 sit-ups. And I repeat the cycle 3 times without breaks. It takes me 15-20 minutes. Then I shadow box, or, stretch, or lift weights, depending on my time and mood.
So, this routine works well and doesn't require much time.
P.S. And have lots of sex! It's kind of "refills" your social and exercise points all at once. :))))