not that far. i used to live in Toronto. if you are young and walk briskly, i would say you could to it in under 40 mins with no stopping. there is not much to see on it really (but it is the worlds longest street). it used to be really run down but they have been working on improving it.

however its just as easy to take the TTC (transit system). Dundas and Yonge is the NORTH tip of the major shopping center that was called the Eatons Center (until Eatons went broke).

its between 2 and 4 stops on the TTC going south. i cannot remember how many exactly but its less than 10 mins on the subway.

you can see a map here
http://www.hoteltravel.com/canada/toronto/eaton_centre_map.htm

if you want to see some other cool Toronto transportation web sites

http://www.urbantoronto.ca/forumdisplay.php?f=8
http://www.toronto.ca/ttc/
http://ttcrider.ca/tips.php