It's been done, but usually isn't pretty. Of course a lot depends on your age, your general fitness level, and your expectations. DS did a half with me when he was 19 (the course included part of his college campus). He didn't train at all, although he was playing intramural basketball at the time. He walked the half with me and did just fine, no problems at all. DH walked his very first half with me a couple of months later. He was, um, a little bit older and not quite as active as DS. He also finished in the timelimit with no problems, but....he was sore for several days after - he really, really felt it. So, while it can be done I wouldn't count on it.