Center Textboxfor with column icon

Question

I’m trying to create a centralized login form, but with the code below the input text is aligned to the left and the icon to the right:

<div class="form-group">
  <div class="row">
    <div class="col-md-4 col-md-offset-4">
      <div class="input-group">
        @Html.TextBoxFor(m => m.Email, new { @class = "form-control", placeholder = "E-mail" })
        @Html.ValidationMessageFor(m => m.Email, "", new { @class = "text-danger" })
        <span class="input-group-addon"><i class="glyphicon glyphicon-envelope"></i></span>
      </div>
    </div>
  </div>
</div>

The input size is set to 280px (bootstrap default in the Site.css file) and this I would not like to change. See in the image below the result, I would like to know how to join the input and the icon, and that the two stay in the center of the column.

I was able to implement it as follows:

            <div class="form-group">
                <div class="row">
                    <div class="col-md-4 col-md-offset-4">
                        <div align="center">
                            <div class="input-group" style="max-width: 280px">
                                @Html.TextBoxFor(m => m.Email, new { @class = "form-control", placeholder = "E-mail" })
                                <span class="input-group-addon"><i class="glyphicon glyphicon-envelope"></i></span>
                            </div>
                            @Html.ValidationMessageFor(m => m.Email, "", new { @class = "text-danger" })
                        </div>
                    </div>
                </div>
            </div>

Jsfiddle