Newbie: off-season volume building starting point

I did my first triathlon (Sprint distance) a couple of weeks ago and I recently read the Mark Allen off-season article that talked about base building (http://www.beginnertriathlete.com/discussion/forums/thread-view.asp?tid=8877&posts=6). In the article, Mark seemed to follow the same general volume building rules that are posted in the training plans on this site: increasing by 10% per week, back off for week 4, etc. When I stay below the heart rate that Mark suggests, I can go much longer than I could when I push myself to the limits. So what should I pick as my starting distances/times? For example, prior to the triathlon, I was swimming a maximum of 1000 meters (the race was 800m). Tonight I swam 1100m (10% increase) but because I was keeping my heart rate low, when I was done, I felt like I could probably do the entire distance again. Should I start with a longer distance or should I build up from where I am?