LINQ - Splitting up a string with maximum length, but not chopping words apart

I have a simple LINQ Extension Method...

    public static IEnumerable<string> SplitOnLength(this string input, int length)
    {
        int index = 0;
        while (index < input.Length)
        {
            if (index + length < input.Length)
                yield return input.Substring(index, length);
            else
                yield return input.Substring(index);

            index += length;
        }
    }

This takes a string, and it chops it up into a collection of strings that do not exceed the given length.

This works well - however I'd like to go further. It chops words in half. I don't need it to understand anything complicated, I just want it to be able to chop a string off 'early' if cutting it at the length would be cutting in the middle of text (basically anything that isn't whitespace).

However I suck at LINQ, so I was wondering if anyone had an idea on how to go about this. I know what I am trying to do, but I'm not sure how to approach it.

So let's say I have the following text.

This is a sample block of text that I would pass through the string splitter.

I call this method SplitOnLength(6) Я бы получил следующее.

• Это
• sa
• mple b
• lock o
• f text
• , которое я
• передал бы
• через
• s
• tring
• splitt
• er.

Я бы предпочел, чтобы он был достаточно умен, чтобы остановиться и больше походить на ..

• Этот
• -
• образец

// плохой пример, так как одно слово превышает максимальную длину, но в реальных сценариях длина будет больше, чем 200.

Кто-нибудь может мне помочь?