It will likely be crowded, it's almost "take a number" to go up (sort of like being in Le Bourgue d'Oisins in August to climb Alpe d'Huez). But hiking up a steep hill uses most of the same muscles used in cycling, so you'll do just fine. Just don't let them talk you into hiking back down - you will definitely hurt yourself if you do! I've heard of guys not being able to walk the next day! They have a gondola that will take you back down. It used to be free (only pay going up), but hiking up Grouse got to be too popular, so now you have to pay both ways. Nice views at the top, especially if it's a clear day, nice restaurant - have a leisurely meal as the sun sets and the lights below come on.
Luis