Howdy CBW - The Wall will be my first ultra, though for me the deciding bit was easy, a few beers on payday and a handy debit card while trawling around t'internet. 
Training up to longer distances - been running the occasional half marathon distance, and a couple of 10 mile + per week, building up to longer as I go, just hard to find the time. Main worry for me is getting used to back-to-back running - after a halfie I usually take a day (or two) out to recover - with two 35 mile runs need to get used to hobbling off on the next day.
Still, inventing some crackers new routes as I go - for New Year's Day I'm planning to run from the lighthouse at the end of the north pier at the mouth of the Tyne to the lighthouse at the end of the south pier using the first runnable river crossing, the Tyne Pedestrian Tunnel, for a total of about 10.5 miles. Tempted to GoPro it... 