You should consider using the gx:Track as the geometry for your paths in which the speed and elevation can be shown in the Elevation profile.
A good example can be found in this article: http://google-latlong.blogspot.com/2010/06/relive-your-hiking-biking-and-other.html
For each vertex you have a time-location pair with a when for the time and gx:coord for the location. You can also add custom metadata at each time-location pair using an ExtendedData element with gx:SimpleArrayData. See track example with ExtendedData.
<gx:SimpleArrayData name="cadence">
<gx:value>86</gx:value>
<gx:value>103</gx:value>
<gx:value>108</gx:value>
<gx:value>113</gx:value>
<gx:value>113</gx:value>
<gx:value>113</gx:value>
<gx:value>113</gx:value>
</gx:SimpleArrayData>
If you use the track icon style the heading is dynamically oriented at the current heading as you animate the path.
UPDATE: as Peter commented, the use of the gx: namespace is a Google extension which works fine with Google products but the KML may not work as expected with non-Google products.