Well, I did it for myself, so I may as well share.
Code:
<thing id="pExpRet" name="Expeditious Retreat" description="Your speed increases by 30 ft." compset="InPlay">
<fieldval field="pMinimum" value="30"/>
<fieldval field="pMaximum" value="30"/>
<fieldval field="pDuration" value="1 minute/level (D)"/>
<tag group="Helper" tag="AdjSpell"/>
<tag group="Helper" tag="NoIncr"/>
<tag group="Helper" tag="AdjNoPlus"/>
<eval phase="First" priority="1000"><![CDATA[
~ If we're not enabled, get out now
doneif (field[pIsOn].value = 0)
~ Add to speed
hero.child[Speed].field[Bonus].value += 30]]></eval>
</thing>