Hey Jake...I think it's cool that you want to get into mountain biking =) I think the lake you're talking about is Lake Chabot...during the summer I run there a lot with the MCHS XC team for training an have yet to go around the whole lake, but group of the guys did it once and they said its 13 miles all the way around. Very ambitious...but I think you could do it.

Good luck w/ buying a bike!