NEW YORK (CNN/Money) - I've recently come into a large sum of money -- about $350,000 -- and I'm wondering what's the best way for me to keep it growing so I can retire at, say, 50, rather than 65. What do you recommend?

-- Eric Jordan, Tacoma, Washington

I'm glad to see that in the excitement of getting a nice $350,000 windfall, you still have the presence of mind to know you must do some planning and investing for the future.

Often, the knee-jerk response from people who have a big lump sum dumped into their lap is to rev up their spending. Before long, their big stash has become a mere pittance and they've made no headway toward securing their future financial security. I applaud you for avoiding that pitfall.

Basically, you're grappling with two issues here: the first is how to invest your money to give your nest egg the best possible chance of growing in value between now and the time you retire.

The second is figuring how much income you can realistically expect to draw from your nest when you do retire-or, put another way, how large must you grow your nest egg in order to get the income you require.

Let's address those issues in order.

Even though you haven't given me your age -- which means I don't know how many years it will be before you turn 50 -- I'm going to assume that you are a long-term investor (i.e., someone with 10 or more years till retirement) and that you're willing to take reasonable risks to make your money grow. Essentially, that means the bulk of your stash should be invested in stocks or stock mutual funds and the rest in bonds or bond mutual funds.

The exact stocks-bonds split can vary from person to person depending on how much one is willing to see his or her portfolio fluctuate in value with the ups and downs of the stock and bond markets.

But, generally, I'd say that a mix of 70 percent stocks and 30 percent bonds would be a good starting point from which more aggressive investors could dial up the percentage in stocks and more conservative types could lean a bit more toward bonds. For more advice on how to arrive at a suitable stocks-bonds mix for you, go to our Asset Allocator tool.

If you feel comfortable assembling a diversified portfolio of stock and bond funds that will reflect your asset mix, then you can check out our Fund Screener for fund candidates. If you're not so confident of your fund picking ability -- or you want to take a simpler route -- then I suggest you consider investing index or life-cycle funds. For more on those options, click here.

As I mentioned earlier, the second issue you've got to grapple with is whether your $350,000 portfolio will be large enough to throw off the amount of income you'll when you retire -- and throw it off long enough so that you won't run out of income before you die. Given ever-increasing lifespans, a person retiring at 50 could easily be talking about a retirement that could last 40 or more years.

I believe that in an era when traditional company pensions are disappearing and Social Security benefits are more likely to be watered down than bulked up, managing an investment portfolio during retirement will be one of the major challenges today's and future retirees will face -- which is why I devote a full chapter to this issue in my new book, "We're Not In Kansas Anymore: Strategies for Retiring Rich In A Totally Changed World".

Just to give you an idea of the challenge we're talking about here, let's use an example.

Assume that you retire at age 50 and by the time you call it a career your $350,000 portfolio has doubled in value to $700,000. That may sound like a huge amount of money -- and it is -- but if you want it to last through a retirement of 40 or more years, you have to keep withdrawals to a relatively small percentage.

How small? Well, we're probably talking an initial withdrawal rate of 4 percent of the portfolio's value -- an amount that would then be increased to keep up with inflation each year. (If you fail to increase your withdrawals with inflation, your purchasing power will decline.)

With a $700,000 portfolio, that would mean an initial withdrawal of just $28,000, or a bit over $2,300 a month. Now, you could certainly withdraw more -- 5 percent, or $35,000 initially; 6 percent, or $42,000; even 10 percent, or $70,000. But here's the problem. The higher your withdrawal rate, the greater the chances that your portfolio will run out of money before you die.

Over the years, researchers have performed computerized simulations involving thousands of possible scenarios with a variety of withdrawal rates and different rates of return. And what they've found is that once you get above an initial inflation-adjusted withdrawal rate of 4 to 5 percent, the odds of running out of money in 30 or more years tends to go up to what most people would consider an unacceptable level -- 20 percent or higher.

By keeping your withdrawal rate down, however, you increase the odds that your portfolio will last 30, 40 or even more years, and there's even the chance your portfolio might grow (in which case, you could increase your withdrawals or leave something for your heirs). For a look at how various withdrawal amounts affect the longevity of your portfolio, click here.

So what does all this have to do with your original question of how to invest your $350,000 so you can retire at 50? Several things.

First, you're going to have to grow that 350 grand into a pretty sizeable nest egg if you hope to pull a decent amount of income from it at retirement.

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That probably means not only investing the bulk of it in equities, but continuing to add new savings along the way (either directly or by investing in 401(k)s, IRAs or other retirement accounts). And once you begin relying on your portfolio for regular income, you'll have to be careful about withdrawals.

You've certainly got a leg up on retirement with your $350,000 windfall. But even with that substantial sum, you still have to do some careful planning and smart investing if you want to live comfortably in retirement, and avoid running out of money before you run out of time.

Walter Updegrave is a senior editor at MONEY Magazine and is the author of "We're Not in Kansas Anymore: Strategies for Retiring Rich in a Totally Changed World." He also answers viewers' questions on CNNfn's Money & Markets at 4:40 PM on Mondays.