Before starting any exercise program consult your physician. Make sure you are healthy enough to participate in a strenuous training program, FIRST.
Although there were some great suggestions, the effects of physical activity, and our body's adaptation to it, are individual in nature. Simply, what works for others, may not work for you. Furthermore, your body is not as strong, limber, or elastic as it once was. So, performing strength movements (of the past) may not have the same effects and may increase the risk of injury. You need to build a foundation of aerobic and anaerobic conditioning, strength, and most importantly flexibility. Flexibility is the most overlooked component of any training program. Most people skip it because it's uncomfortable, it hurts and it takes time. Big mistake!! Flexibility increases the range of motion in joints, burns calories, lengthens muscle fibers and promotes the growth of lean muscle tissue. Our ability to move without restriction can lead to an increase in overall strength, size, balance and power. The more lean mass you can develop, the more calories you will burn.
Diets DO NOT work!! Eating healthy is a lifestyle change. The more active you are, the more calories your body needs. Making healthier choices does not mean you need to limit calories or sacrifice taste. It means giving your body what it needs, when it needs it without the empty calories. Breakfast is the most important meal of the day. Don't skip it!! You should eat every 2-3 hours and a total of 5 or 6 times per day (3 main meals and 2 or 3 small meals). FYI, not eating anything after 8pm, is a myth. You should never go to sleep hungry. If your eating healthier, a late snack is quite beneficial. Our bodies burn the most calories while we sleep. So be sure to get a full 8 hours every night. Also, stay hydrated, drink plenty of water.
My two-cents, as someone on the plus side of 40, and a strength and conditioning coach:
ANY activity is better than inactivity. Walk, jog, bike, run, jump rope, whatever motivates you to move....do it. Even if it for 10-minutes a day. I prefer Power Yoga. I do 30-minutes, a 2-3 times a week. It's the perfect combination of strength, power, balance, and flexibility. It took me a while to perform many of the balances and poses. As a former collegiate athlete, it wasn't as easy as it looked. But if you push through it, the benefits far outweigh the early discomfort. I feel the best I ever have.
Always warm-up before you perform any exercise program. You need to get the body warm, the blood and oxygen flowing. A dynamic warm up is a great way to kick start your body's caloric furnace. A dynamic warm up gets you moving and stretches to muscles, in a variety of planes, in preparation for more strenuous activity.
Start with body weight movements (various push ups, squats, and planks) will help you to re-develop basic strength in the major muscles and your core (abdominals and lower back). As your body adapts (usually 4-6 weeks), add reps, resistance or increase the intensity. Don't over-do it. Start with one exercise for each major muscle, perform 1-2 sets of 8-10 reps. Rest as long as you feel necessary between sets. As you progress, decrease the rest period, to increase your level of aerobic conditioning and keep the calorie burning cranked up. If your workouts are more then 40-minutes per session, your wasting your time. There are more beneficial and efficient ways to get results.
Always stretch large muscles to small at the completion of your strength training sessions. Your body is primed for static (old school) stretches at this point. It takes five-minutes to do. Never just walk out. Let your heart rate and core body temp come down while performing some basic stretches. Foam rollers and a lacrosse ball are great here too, but if you don't know what your doing, you could do more harm then good.
Keep it simple....eat smart, hydrate, stay active, rest.
Good luck!!