trail running and ultramarathons

Yesterday I was listening to a podcast from the Marathon Show (with Joe Taricani -- I love that show!) during which Joe was interviewing someone who runs ultramarathons.  The runner's enthusiasm for the sport inspired me to look into them for myself.  Later in the day I was looking at the California International Marathon's website for training plans and pace leaders information.  I noticed the pacers all have done many marathons, but they all were even more into ultramarathons.  It's quite impressive to see so many runners having run 2 or 3 times more ultras than marathons during their career.  It made me wonder if I'm missing something.

I'm not talking about a 50 or 100 miler for myself, but maybe topping out at a 50K (roughly 31 miles).  That isn't a whole lot more than the standard 26.2 of a marathon, but if you're like me then you know how hard the race becomes after about 20-22 miles.  Today, there's no way I could do a 50K but I'm wondering if maybe next Fall I could. 

I haven't done much off-road racing so I'm looking into that first.  Have you switched from road racing to trails?  Have advice to share with myself and others who may be wanting to try it?  Maybe you have distances or specific races you'd recommend?  A couple of people in my running group have said in the past that an ultra is a completely different experience and something a few of us should look into.  I'm curious!