incognito, thats exactly what the reason I started attending events this year, after riding the same trails for 8 years. Seriously, have a look at Trail Break (mainly SE) and the events sections in here and on BikeMagic. I've ridden 2 this year so far (3 to go) and although you get a race number you don't have to take it too seriously - lets face it most of us won't get on the podium. There are all shapes and sizes of riders and bikes, nothing to fret about.
Personally I've found it great motivation to squeeze in extra "Training" rides (so I can ride the distances and challange myself personally by getting a decent time), I've lost 1/2 stone since Jan and my motivation / experience / fitnes is shooting up.
Fyi, I did a Trail Break 40k in Marlborough, the recent Gorrick (54k half distance) at Swinley and have entered the Whyte 50k at Surrey & SITS.
I've also got included in other rides by way of getting to know other riders in Wales, Peaks and in June Dalby & Gisburn.
Personally I've loved it.