- to walk = lopen
- to run/to jog (as a sport or exercise) = hardlopen
- to run (if it's not as a sport, e.g. if you have to catch a train) = rennen
- to stroll = wandelen
Indeed in cases like Hij liep de marathon (He ran the marathon) or Op het EK loopt hij de 100 meter (At the European Championships he will run the 100 meters) you would expect hardlopen to be used, but here lopen is used for some reason. (Maybe it's just a shortening of hardlopen because it's very obvious that that is meant here.)
BTW the explanation above applies to Dutch spoken in the Netherlands, in Belgium this is used differently, but I'm not going to include that, I guess it's confusing enough as it is. :)
In scandinavian, "løbe" (very similar to 'lopen') means to run (and only run). Maybe that is why 'lopen' also sticks to 'running'.
I also got confused when i participated in the Batavierenrace (relay race in NL) and they always(!) said 'lopers' and 'lopen' about runners and running. Never 'rennen' or 'hardlopen'.