Eight in every 10 people aged 40 to 60 in England are overweight, drink too much or get too little exercise, the government body warns.
PHE wants people to turn over a new leaf in 2017 and make a pledge to get fit.
Health officials say the "sandwich generation" of people caring for children and ageing parents do not take enough time to look after themselves.
We are living longer, but are in poorer health because we store up problems as we age. The campaign's clinical adviser, Prof Muir Gray, said it was about trying to make people have a different attitude to an "environmental problem".
"Modern life is dramatically different to even 30 years ago," Prof Gray told Radio 4's Today programme. "People now drive to work and sit at work."
By taking action in mid-life... you can reduce your risk not only of type 2 diabetes, which is a preventable condition, but you can also reduce your risk of dementia and disability and, being a burden to your family,' he added.
Many people no longer recognise what a healthy body weight looks like, say the officials - and obesity, which greatly increases the risk of type 2 diabetes, is increasingly considered normal.
Although physical fitness is not the equivalent of overall health, along with a good diet, it is a great start. By now, the health risks to our bodies and minds of getting no exercise have become well established. Adding years to our life expectancy is not desirable if we are talking about extra years as a physical or mental cripple. An alarming thought is that we will increase our risk for mental degradation by being sedentary. But that is what modern research points to.
BBC.com also reports that being unfit in middle years hastens brain deterioration, citing a study that was published in the journal of the American Academy of Neurology. The study found that being unfit at age 40 correlated with a reduced brain volume at age 60, and that the brain shrinkage denotes accelerated brain ageing. 1,583 participants who were free of dementia or heart disease were studied, and then again twenty years later. MRI brain scans were given along with a treadmill test. As quoted in the article, lead researcher Dr Nicole Spartano, of the Boston University School of Medicine opined,
"While not yet studied on a large scale, these results suggest that fitness in middle age may be particularly important for the many millions of people around the world who already have evidence of heart disease."
Moderate alcohol consumption is a strategy employed by some of us as a coping mechanism - alcohol can take the edge off of stress and anxiety. But immoderate alcohol use can cause more problems that it solves - it can interfere with our normal daily functioning, and enable us to ignore issues that need to be addressed. Cutting back on the amount of alcohol consumed can in itself solve problems, not the least of which is our health concerns as we enter our middle years.
Abusing alcohol in our youth can seem to be without consequence, but the partying lifestyle is not sustainable. Our wake-up call may come in the form of failing health in later years, forcing us to confront our poor choices. Is now the time to forge a new path in the new year? Yes! You can ask a friend or family member to hold you accountable in turning a new leaf. You don't have to go it alone - your loved ones can aid you in remaining firmly resolute. The worst you can do is to beat yourself up for past failures. No matter how many breaths and heartbeats you have left, it is up to you to make the best of it, from this day forward.
Know that willpower is not a commodity of limited availability, to be conserved for emergency use only, but more like a muscle group that responds with additional capacity as you exercise it. You can start small and think big - bad habits can be overcome incrementally. And of course for some people, like alcoholics, diabetics, and pre-diabetics - the only solution is to stop drinking alcohol altogether.