They say life is short, but on average, humans are living longer than ever before, largely thanks to advances in healthcare and technology. This renewed longevity also means retirement can be years or decades longer than it used to be, for many. It's not unheard of for a retiring 60-year-old to live another 20 or 30 years; that's the good news.

But there are challenges that accompany a longer retirement. What will you do in this next phase of your life? While some know what they won't do -- work -- others may have a vague idea of how they'll spend their time. But without a plan for the long term, retirees may be unprepared for how the last act of their life unfolds.

What if you were to think about retirement more specifically as your last several decades, or even gave it a day tally? The average retiree will enjoy 8,000 days, about 22 years, in retirement, according to a recent study by Massachusetts Institute of Technology (MIT) AgeLab. Its "8,000 Days of Retirement" research is part of a look at four phases of life, with the goal of getting us to think about how we can plan and transition through each phase successfully. It also breaks down four stages of retirement to show you how you should be progressing along to make the best of your golden days, or decades.

A senior man and woman smiling and toasting with glasses of white wine at a party

Retirement is a journey, not a destination. Here's how to chart your course. Image source: Getty Images.

Phases in life, and in retirement

MIT AgeLab breaks our lives down into four equal parts, each lasting 8,000 days -- roughly 22 years. The learning phase is from birth through college. The growing phase is from our 20s to mid-40s, when we start families and develop our careers. The maturing years are our mid-40s to mid-60s, when we're in peak earning years and buying bigger homes. The exploring years follow the working years, when our retirement journey begins.

Thinking of retirement as the beginning of a new journey and not as a conclusion can help you become more optimistic about how you'll spend this time. To help retirees think about this new beginning, MIT AgeLab broke retirement itself down into four phases.

1. The honeymoon phase

In any new phase of our lives -- marriage, children, divorce, and retirement -- change can be daunting. The anxiety of something new, and the unfamiliarity of what lies ahead nags at us, and the daily routine we grew so familiar with is gone.

Finding a mentor early on in retirement can help. When recent college graduates land their first job, they are encouraged to seek out a mentor, so why not find a retirement mentor who can show you how it's done? The AgeLab suggests finding someone who is thriving in retirement and ask them to lunch -- retirees love lunch -- to learn about the challenges they faced and mistakes they made, and how they overcame them. Strengthening social bonds by developing a mentorship is a secondary benefit.

2. The big decision phase

Once you're in the swing of retirement, it's time to make bigger decisions like where to live. Is aging in place an option or will you require something different, perhaps with more accessibility or less maintenance?

Think about the ease of getting around the house as you age, but also moving around town too, considering available transit and transportation services. It's a good idea to start thinking about housing now, while there's time to make home improvements or plan to downsize.

This is the phase where we begin pondering our place in the world: What's our purpose? Finding meaning and a reason to get up in the morning will keep you from feeling listless or antsy, which can drive retirees to abandon retirement to go back to work. I had a client once who spent 30 years on Wall Street and retired, only to return to work at another bank within six months. He didn't have a plan for how he would spend his time, so he became bored.

Websites like Encore.org and LifeReimagined.AARP.org can help you find a second act in life. Think not only about what hobbies and activities you enjoy now, but also about how you can still enjoy them in the future. How might aging affect your abilities? How could you continue your interests if your functionality becomes limited? Think about how you can prepare now to live a fulfilling life in your later years, such as choosing a place to live with ample public transportation options, or a town with a thriving senior center.

3. The longevity phase

In the latter part of retirement, our age begins to show. Our cognitive abilities, health, and mobility all start declining, while our financial resources may be lessened.

Here it's best to get organized. Get all the necessary estate documents in place, such as a power of attorney. Organize your finances or consider the help of a financial advisor.

Think about long-term care, and if you or your spouse might require it. Evaluate whether you or your spouse would be willing and able to be a caregiver for the other, and if not, start looking for long-term care providers, just in case. You should also put together a list of more general maintenance professionals including a landscaper, a housecleaner, a plumber, a home-repair person and even an occasional driver.

4. The solo phase

A sudden and unexpected loss of a partner can immediately change our financial picture. Surprisingly, 53% of Americans don't plan for this. You need to plan for life alone, even if you're married now. Get a team in place for caregiving, helping around the house, and managing finances.

The report also encourages retirees in this phase to start a new journey when the time is right. It may be helpful to find a mentor -- a widow or widower who can help you navigate this phase. Keep up your social connections, seek out those with common interests, and gradually start building a new self.

If we only think of retirement as the end of work, we are missing so much more. If we think of retirement as the beginning of a new journey, or the next 8,000 days of our life, then we start to think about the possibilities and plan for the opportunities and challenges that may await us -- all of which can help us transition through each of the four phases of retirement.