Put a space before each capital letter

A quick macro would serve you best here even if you decide not to retain it for future use.

Tap Alt+F11 and when the VBA opens, immediately use the pull-down menus to Insert, Module. Paste the following into the new pane titled something like BooK1 - Module1 (Code),

Sub mcr_Fix_Case()

    Dim r As Range, s As String, c As Long

    For Each r In Selection

        s = vbNullString

        For c = 1 To Len(r.Value)

            s = s & IIf(Asc(Mid(r.Value, c, 1)) > 64 And _

Asc(Mid(r.Value, c, 1)) < 91, _

Chr(32), vbNullString) & Mid(r.Value, c, 1)

Next c

s = Trim(s)

        r = Left(s, 1) & Right(LCase(s), Len(s) - 1)

    Next r

End Sub

Tap Alt+Q to return to your worksheet. First select teh cells containing the run-on sentences then tap Alt+F8 to open the Macros dialog and Run the macro. It will convert every cell you selected from this,

IHaveSentencesThatAreAllRunTogetherAndEachNewWordIsACapitalLetter.

... to this,

I have sentences that are all run together and each new word is a capital letter.

The best and quickest way is Macro, as suggested by Jeepad.

I am trying to give one alternate (Non Macro Solution). Assuming your text is in H7, then give this formula at I7 and drag it right side till you start getting blank cell (or say till 200 columns).

=IFERROR(IF(AND(CODE(MID($H7,COLUMN(A7),1))<91,CODE(MID($H7,COLUMN(A7),1))>64)," "&CHAR(CODE(MID($H7,COLUMN(A7),1))+32),MID($H7,COLUMN(A7),1)),"")&J7

Your result will be at I7, you may hide the other columns.

You may also drag down this formula.