I started to write out updates to the rule to be triggered off a change in the TowelMan switch and put conditional actions, but I'm not sure that will work the way you need it to, hopefully someone else can come up with a way to keep this in the same rule.
The best I can offer is to leave the rule the way it is and setup a second rule triggered by the TowelMan turning off, resulting in actions to cancel any actions from the rule you have above (I'm pretty sure that is possible...), and turning the towel rail off.
I can't claim much experience in that myself, but I did play with Bryan's Kitchen timer app. In searching for it, I found another timer app that he created that may be what you are after:
Here's the Kitchen Timer in case you are interested: