I just helped somebody with this, they wanted to use their BT .ICS calendar workout feed and have that updated to their IPad. You cannot directly add a URL to the Ipad's calendar - I tried. The best way is to do this via ITunes.
To sync your calendars using iTunes:
With iTunes open on your desktop, connect your iPhone, iPod, or iPad to your computer.
Click the Info tab (or the Contacts tab), and then select the “Sync iCal Calendars” checkbox.
Select whether you want to sync all your calendars or only the calendars you select.
Select any other options you want, and then click Apply.
Your calendar information (YOUR WORKOUTS) is updated each time you connect your iPhone, iPod, or iPad to your computer.